Turn control apparatus for a vehicle

ABSTRACT

A turn control apparatus for a vehicle is applied to a four-wheeled automobile. When the vehicle turns while being braked, the outside front and inside rear wheels viewed in the vehicle turn direction are selected from the vehicle wheels as two target wheels to be controlled. The braking force on one target wheel to be controlled is increased in accordance with the turning condition of vehicle, while that of the other target wheel to be controlled is decreased.

TECHNICAL FIELD

The present invention relates to a turn control apparatus for a vehicle,which stabilizes turn behavior of vehicle by controlling the brakingforce on the wheel thereof when a vehicle is turned.

BACKGROUND ART

Turn control apparatuses for a vehicle have been disclosed, it, forexample, in Unexamined Japanese Patent Publication No. 1-237252 andUnexamined Japanese Patent Publication No. 3-143757. In the controlapparatus disclosed in the former Publication, when a vehicle turnswhile being braked, the braking force on the right and left rear wheelsis increased or decreased according to the yaw angular velocity of thevehicle. Thereby, a yaw moment can be produced on the vehicle to preventa spin thereof. In the control apparatus in the latter Publication, thebraking force on the front and rear wheels is controlled according tothe turning condition of the vehicle when the vehicle turns while beingbraked. Thereby, a yaw moment can be produced on the vehicle to assist aturning thereof.

For the control apparatus in the former Publication, the braking forceon only the right and left rear wheels of the vehicle is controlled, sothat the yaw moment generated on the vehicle by this control isrelatively small. Therefore, there is a possibility that a spin of thevehicle cannot be prevented effectively depending on the turningcondition of the vehicle.

For the control apparatus in the latter Publication, the braking forceon all of four wheels of the vehicle are controlled. Therefore, even ifthe driver attempts to apply a stronger braking force to the vehicleduring the control, the driver's intention is not reflected. For thisreason, this control apparatus is not preferable in view of safety whena fault of the control apparatus during the control is considered.

An object of the present invention is to provide a turn controlapparatus for a vehicle, which reliably stabilizes the turn behavior ofthe vehicle when the vehicle is turned, while fully maintaining thesafety of the vehicle.

DISCLOSURE OF THE INVENTION

The above object is achieved by a turn control apparatus for a vehiclein accordance with the present invention. The turn control apparatus fora vehicle comprises determining means for determining the vehicleturning condition. The determining means includes turn detecting meansfor detecting a turn of the vehicle and outputting a turn signalindicative of a turn direction of the vehicle and braking detectingmeans for detecting the braking of the vehicle and outputting a brakingsignal indicative of a braking condition of the vehicle. The turncontrol apparatus further comprises selecting means for selecting theoutside front and inside rear wheels viewed in the vehicle turndirection as two target wheels to be controlled based on the turn signalfrom the turn detecting means, and first braking control means forincreasing the braking force on one target wheel to be controlled anddecreasing the braking force on the other target wheel to be controlledin accordance with the vehicle turning condition determined by thedetermining means when the vehicle is braked.

According to the above-described turn control apparatus, when thevehicle turns while being braked, the outside front wheel and insiderear wheels viewed in the vehicle turn direction are selected as thetarget wheels to be controlled, and the braking forces on these targetwheels to be controlled are increased and decreased in accordance withthe vehicle turning condition. Therefore, a turning yaw moment orrestoration moment necessary for the vehicle is given to the vehicle.When the braking forces on the target wheels to be controlled, or theoutside front wheel and inside rear wheel viewed in the vehicle turndirection, are controlled, the turning moment or restoration moment tobe given to the vehicle is generated most effectively, by which the turnbehavior of the vehicle is stabilized. The number of target wheels to becontrolled is few, being two, so that the braking forces on the wheelscan easily be controlled.

In this case, the braking forces on the wheels other than the targetwheels to be controlled, or the noncontrolled wheels, are determined bythe depressing operation of a brake pedal performed by the driver. Sincethe braking forces on the noncontrolled wheels are controlled by thedriver's intention, a sense of strangeness caused by braking is notgiven to the driver, and also safety is fully ensured.

The first braking control means sets the increase amount and decreaseamount of braking force on the target wheels to be controlled at thesame value in terms of absolute value. In this case, the braking forceof the whole vehicle is not changed, and the braking feeling of vehicleis not adversely affected.

The first braking control means comprises vehicle condition detectingmeans for detecting a vehicle operating condition and manipulationcondition, and setting means for setting the increase amount anddecrease amount of braking force on the target wheels to be controlledbased on the detected vehicle operating condition and manipulationcondition. More specifically, the vehicle condition detecting meansincludes means for setting a target yaw rate of vehicle, and the settingmeans can set the increase amount and decrease amount of braking forceon the target wheels to be controlled based on the yaw rate deviationbetween the target yaw rate and the actual yaw rate of vehicle. Further,the setting means can set the increase amount and decrease amount ofbraking force on the target wheels to be controlled based on thederivative of yaw rate deviation in addition to the yaw rate deviation.In this case, the increase amount and decrease amount of braking forceon the target wheels to be controlled are set based on the yaw ratedeviation (the derivative of yaw rate deviation) of vehicle exactlyreflecting the actual operating condition of vehicle, so that vehicleturn control, that is, vehicle yaw moment control is carried out veryfinely and stably.

The aforementioned turn detecting means can include a yaw rate sensorfor detecting the yaw rate of vehicle and discriminating means fordiscriminating the turn direction of vehicle based on the output of theyaw rate sensor. The output of the yaw rate sensor indicates the actualadvancing direction of vehicle, that is, the turn direction of vehicle,so that the vehicle turn direction is discriminated exactly.

The first braking control means includes brake lines of a so-calledcross-piping form. In this case, the braking line for the target wheelsto be controlled and the brake line for the noncontrolled wheels areindependent of each other, providing great safety.

The turn control apparatus for the vehicle can further comprise secondbraking control mean for increasing the braking force on one of thetarget wheels to be controlled in accordance with the vehicle turningcondition when the vehicle turns while being not braked. In this case,even if the vehicle is not braked, the braking force on one of thetarget wheels to be controlled is increased, so that the turning momentor restoration moment can be given to the vehicle.

The first and second braking control means include vehicle conditiondetecting means for detecting a vehicle operating condition andmanipulation condition, computing means for computing a requiredcontrolled variable for the vehicle turn based on the detected vehicleoperating condition and manipulation condition, and setting means forsetting the increase amount and decrease amount of braking force on thetarget wheels to be controlled based on the required controlledvariable. In this case, a gain for computing the required controlvariable when the vehicle is braked is different from that when thevehicle is not braked, and the gain is set so as to be larger when thevehicle is not braked than when the vehicle is braked.

When the vehicle turns while being not braked, the increase amount ofbraking force on one target wheel to be controlled becomes larger ascompared with the case where the vehicle turns while being braked.Therefore, even if the braking force on the other target wheel to becontrolled is not decreased, the turning moment or restoration momentcan fully be given to the vehicle.

The first braking control means includes a hydraulic pressure controlvalve for controlling a brake pressure of wheel brake for each wheel bycooperating with the corresponding wheel brake, a pump for supplying apressurized fluid toward the wheel brake for each wheel, and holdingmeans for changing over the hydraulic pressure control valve to hold thebrake pressure of wheel brake for the noncontrolled wheel. In this case,even if the pump is actuated to supply the pressurized fluid toward thewheel brakes for the target wheels to be controlled, the wheel brake forthe noncontrolled wheel is not subjected to the discharge pressure fromthe pump, so that the braking force on the noncontrolled wheel is notincreased.

The braking detecting means includes augmented depression detectingmeans for detecting augmented depression of vehicle brake pedal andoutputting an augmented depression signal, and the first braking controlmeans includes a hydraulic pressure control valve which is changed overfrom the noncontrol position to control the brake pressure of thecorresponding wheel brake by cooperating with the wheel brake for eachwheel, and returning means for returning all of the hydraulic pressurecontrol valves to the noncontrol position when the augmented depressionsignal is received. In this case, if augmented depression of brake pedalis performed, all hydraulic pressure control valves are returned to thenoncontrol position, and the braking force on each wheel is controlledby the operator's intention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a brake system for carrying out yaw momentcontrol of a vehicle;

FIG. 2 is a block diagram showing the way an electronic control unit(ECU) in the brake system shown in FIG. 1 is connected to varioussensors and a hydraulic unit (HU);

FIG. 3 is a functional block diagram for schematically illustrating thefunction of the ECU;

FIG. 4 is a flowchart showing a main routine executed by the ECU;

FIG. 5 is a graph showing a change in steering wheel angle θ with timewhen a steering wheel is manipulated;

FIG. 6 is a flowchart showing a setting routine for part in Step S2 ofFIG. 4;

FIG. 7 is a view showing the details of a turn determination sectionshown in FIG. 3;

FIG. 8 is a flowchart showing the details of a determination routineexecuted in the turn determination section shown in FIG. 3;

FIG. 9 is a diagram showing the details of a target yaw rate computingsection shown in FIG. 3;

FIG. 10 is a diagram showing the details of a required yaw momentcomputing section shown in FIG. 3;

FIG. 11 is a flowchart showing a required yaw moment computationroutine;

FIG. 12 is a block diagram for determining a proportional gain for thecomputation of a required yaw moment;

FIG. 13 is a flowchart showing a correction factor computing routine forthe computation of a proportional gain;

FIG. 14 is a graph showing the relation between vehicle body velocityand reference lateral acceleration;

FIG. 15 is a view for illustrating the turn behavior of a vehicle inrelation to a gravity-center slip angle β when the vehicle is turned;

FIG. 16 is a flowchart showing a correction factor computation routinein relation to a proportional gain and integral gain;

FIG. 17 is a graph showing the relation between gravity-center slipangular velocity and reference correction factor;

FIG. 18 is a block diagram for computing the vibration component of yawrate;

FIG. 19 is a flowchart showing a correction factor computation routinein relation to a proportional gain;

FIG. 20 is a graph showing the relation between vibration component ofyaw rate and correction factor;

FIG. 21 is a block diagram for determining an integral gain for thecomputation of a required yaw moment;

FIG. 22 is a graph showing the relation between absolute value ofsteering wheel angle θ and correction factor of integral gain;

FIG. 23 is a diagram showing the details of a yaw moment control sectionshown in FIG. 3;

FIG. 24 is a diagram showing the details of an on-off determinationsection shown in FIG. 23;

FIG. 25 is a graph showing a setting reference for control executionflag;

FIG. 26 is a flowchart showing a control mode selection routine;

FIG. 27 is a time chart showing the relations between a control mode,actuation mode, and pulse width;

FIG. 28 is a flowchart showing an actuation mode setting routine;

FIG. 29 is a block diagram showing the details of an inhibitory sectionshown in FIG. 23;

FIG. 30 is a flowchart showing a setting routine for one inhibiting flagin relation to the inhibitory section;

FIG. 31 is a flowchart showing a setting routine for another inhibitingflag in relation to the inhibitory section;

FIG. 32 is a graph showing the relation between required yaw moment andallowable slip factor;

FIG. 33 is a graph showing the relation between required yaw moment andallowable slip factor after the start of brake pressure control carriedout by an ABS;

FIG. 34 is a flowchart showing a setting routine for still anotherinhibiting flag in relation to the inhibitory section;

FIG. 35 is a block diagram showing the details of a forced-modificationsection shown in FIG. 23;

FIG. 36 is a block diagram showing part of the actuation determinationsection shown in FIG. 23;

FIG. 37 is a block diagram showing part of the actuation determinationsection shown in FIG. 23;

FIG. 38 is a block diagram showing part of the actuation determinationsection shown in FIG. 23;

FIG. 39 is a block diagram showing part of the actuation determinationsection shown in FIG. 23;

FIG. 40 is a flowchart showing an ABS cooperation routine;

FIG. 41 is a block diagram showing the details of a selecting sectionshown in FIG. 3;

FIG. 42 is a flowchart showing a drive signal initial setting routine;

FIG. 43 is a flowchart showing an actuation routines;

FIG. 44 is a time chart showing the relations between actuation mode,pulse width, actual actuation mode, and actual pulse width;

FIG. 45 is a graph showing braking force and cornering forcecharacteristics versus the slip factor of wheel;

FIG. 46 is a view for illustrating the execution result of yaw momentcontrol when the right turn of vehicle is in an understeer state duringthe braking of vehicle;

FIG. 47 is a view for illustrating the execution result of yaw momentcontrol when the right turn of vehicle is in an oversteer state duringthe braking of vehicle;

FIG. 48 is a view for illustrating the execution result of yaw momentcontrol when the vehicle is not braked and the vehicle is in acountersteer state;

FIG. 49 is a view for illustrating the execution result of yaw momentcontrol when the vehicle is in a critical braking state and countersteerstate;

FIG. 50 is a view for illustrating the execution result of yaw momentcontrol when brake pressure control is carried out by an ABS and whenthe right turn of vehicle is in an understeer state;

FIG. 51 is a view for illustrating the execution result of yaw momentcontrol when brake pressure control is carried out by an ABS and whenthe right turn of vehicle is in an oversteer state;

FIG. 52 is a view for illustrating the execution result of yaw momentcontrol when brake pressure control is carried out by an ABS and whenthe right turn of vehicle is in an understeer state; and

FIG. 53 is a view for illustrating the execution result of yaw momentcontrol when brake pressure control is carried out by an ABS and whenthe right turn of vehicle is in an oversteer state.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to FIG. 1, which schematically shows an automotive brakesystem, the brake system comprises a tandem master cylinder 1, which isconnected to a brake pedal 2 through a vacuum brake booster 2. Themaster cylinder 1 has a pair of pressure chambers, each of which isconnected to a reservoir 4. Main brake lines 5 and 6 extend from thepair of pressure chambers each into a hydraulic unit (HU) 7. In thehydraulic unit 7, the main brake lines branch into a pair of branchbrake lines each.

Branch brake lines 8 and 9, which diverge from the main brake line 5,are connected to wheel brakes (not shown) for front-left and rear-rightwheels FW_(L) and RW_(R), respectively. Branch brake lines 10 and 11,which diverge from the main brake line 6, are connected to wheel brakes(not shown) for front-right and rear-left wheels FW_(R) and RW_(L),respectively. Thus, the wheel brakes for the four wheels of the vehicleare connected to the tandem master cylinder 1 through the brake lines ofa so-called cross-piping form.

A solenoid valve unit is inserted in each of the branch brake lines 8 to11. Each solenoid valve unit has an inlet valve 12 and an outlet valve13. A proportional valve (PV) is interposed between the rear wheel brakeand the inlet valve 12 of the solenoid valve unit corresponding thereto.

For the solenoid valve units for the branch brake lines 8 and 9, abranch return line 14 extends from the outlet valve 13 of the respectivesolenoid valve units. These branch return lines 14 are connected to onemain return line 14_(M), which is connected to the reservoir 4. For thesolenoid valve units for the branch brake lines 10 and 11 as well, abranch return line 15 extends from the outlet valve 13 of the respectivesolenoid valve units. These branch return lines 15 are connected to onemain return line 15_(M), which is connected to the reservoir 4.Therefore, the brake pressure (pressure in the wheel brake) of eachwheel can be controlled by opening/closing the inlet and outlet valves12 and 13 of the corresponding solenoid valve unit.

Pumps 16 and 17 are connected to the main brake lines 6 and 5,respectively, and check valves are interposed between a discharge portof the pump 16 and the main brake line 6 and between a discharge port ofthe pump 17 and the main brake line 5. These check valves permit onlythe flow of pressure oil from the pump to the main brake line. The pumps16 and 17 are connected to a common motor 18, which drives the pumps 16and 17 synchronously. The intake ports of the pumps 16 and 17 areconnected to the aforementioned main return lines 15_(M) and 14_(M),respectively.

Cutoff valves 19 and 20, formed of solenoid valves, are inserted in themain brake lines 5 and 6, respectively. These cutoff valves arepositioned on the upstream side of the pumps 16 and 17. Moreover, themain brake lines 5 and 6 include bypass lines that bypass the cutoffvalves 19 and 20, respectively, and are provided with a relief valve 21each. The cutoff valves 19 and 20 constitute a cutoff valve unit (CVU)22.

The aforementioned inlet and outlet valves 12 and 13 of the solenoidvalve units, cutoff valves 19 and 20, and motor 18 are connectedelectrically to an electronic control unit (ECU) 23. More specifically,the ECU 23 includes a microprocessor, memories such as RAM and ROM,input and output interfaces, etc. The output interface is connectedelectrically to the valves 12, 13, 19 and 20 and motor 18. The inputinterface of the ECU 23 is connected electrically to wheel velocitysensors 24, which are attached individually to the wheels, and arotational speed sensor 25 for detecting the rotational speed of themotor 18. For ease of illustration in FIG. 1, the connections betweenthe motor 18 and the ECU 23 and between the rotational speed sensor 25and the ECU 23 are omitted.

As shown in FIG. 2, the input interface of the ECU 23 is connectedelectrically to a steering wheel angle sensor 26, pedal stroke sensor27, longitudinal acceleration sensor (longitudinal G sensor) 28, lateralacceleration sensor (lateral G sensor) 29, and yaw rate sensor 30, aswell as the wheel velocity sensor 24 and the rotational speed sensor 25.The steering wheel angle sensor 26 detects the steering angle of asteering wheel of a vehicle, that is, the steering wheel angle. Thepedal stroke sensor 27 detects the depth of depression of the brakepedal 3, that is, the pedal stroke. The longitudinal and lateral Gsensors 28 and 29 detect longitudinal and lateral accelerations that actin the longitudinal and lateral directions of the vehicle. The yaw ratesensor 30 detects the vehicle angular velocity around a vertical axis,that is, the yaw angular velocity.

The ECU 23 receives the output signals of the aforementioned varioussensors, and controls the operations of the HU 7 and the CVC 20 based onthese output signals and the various vehicle motion control operations.As shown in the block for the ECU 23 in FIG. 2, the vehicle motioncontrol operations include yaw moment control, traction control,anti-skid brake system control (ABS control), and braking forceallocation control.

FIGS. 3 and 4 show a block diagram and main routine, respectively,associated with the yaw moment control of the above-described functionsof the ECU 23. The control period T of the main loop is set to, forexample, 8 msec.

When the output signals from the aforementioned various sensors aresupplied to the ECU 23, the output signals, that is, the sensor signalsare filtered in the ECU 23 (block 32 in FIG. 3). For filtering, arecursion type primary low-pass filter is used. Unless otherwisespecified, a recursion type primary low-pass filter is also used in thefiltering processes mentioned later.

Next, the ECU 23 reads the filtered sensor signals, that is, wheelvelocities V_(W) (i), steering wheel angle θ, pedal stroke S_(t),longitudinal acceleration G_(X) (longitudinal G_(X)), lateralacceleration G_(Y) (lateral G_(Y)), and yaw rate γ (Step S1 in FIG. 4),and computes information indicative of the vehicle operating conditionand information for judging driver's manipulations (Step S2). Thecharacter i in the wheel velocity V_(W) (i) indicates the number foridentifying the wheel of the vehicle. That is, V_(W) (1), V_(W) (2),V_(W) (3), and V_(W) (4) indicate the wheel velocities of front-leftwheel, front-right wheel, rear-left wheel, and rear-right wheel,respectively. In the description to follow, the reference character (i)will be used in the same sense.

In FIG. 3, Step S2 is executed in the operation blocks 34 and 36.Specifically, in the operation block 34, the vehicle operating conditionis computed based on the wheel velocities V_(W) (i), longitudinal G_(X),lateral G_(Y), and yaw rate γ. In the operation block 36, the driver'smanipulations on the steering wheel and brake pedal is judged based onthe steering wheel angle θ and pedal stroke S_(t).

The vehicle operating condition and driver's manipulations will now bedescribed in detail.

Vehicle operating condition

A: Reference wheel velocity

First, a reference wheel velocity V_(S) is selected among the wheelvelocities V_(W) (i) in the ECU 23. As the reference wheel velocityV_(S), a wheel that is not susceptible to a slip in relation to thebraking force control of the wheel is selected. More specifically, thevelocity V_(W) of the faster driven wheel is selected as the referencewheel velocity V_(S) when the vehicle is not braked. Contrarily, whenthe vehicle is braked, the velocity V_(W) of the fastest wheel of thewheel velocities V_(W) (i) is set as the reference wheel velocity V_(S).As described later, in the ECU 23, whether the vehicle is braked or notis determined by a brake flag F_(b).

B: Vehicle body velocity

Next, the ECU 23 computes the gravity-center velocity of the vehiclefrom the reference wheel velocity V_(S), and determines the vehicle bodyvelocity V_(B) based on this gravity-center velocity. In computing thegravity-center velocity, the inside and outside wheel velocities and thevelocity ratio between front and rear wheels when the vehicle is turningare considered.

When the front and rear treads of the vehicle are denoted by T_(f) andT_(r), respectively, the inside-outside wheel velocity differencesΔV_(IF) and ΔV_(IR) between the front wheels and between the rear wheelsare expressed as the product of yaw rate γ and tread as seen from thefollowing equation.

    ΔV.sub.IF =γ×T.sub.f                     (1)

    ΔV.sub.IR =γ×T.sub.r                     (2)

Therefore, the average of the right-left velocity difference of thewhole vehicle, that is, the average inside-outside wheel velocitydifference ΔV_(IA) is expressed as

    ΔV.sub.IA γ×(T.sub.f +T.sub.r)/2         (3)

If the center of turn of the vehicle is on an extension of the rear axleand when the vehicle is turning clockwise, the front-rear wheel velocityratios R_(VR) and R_(VL) on the right- and left-wheel sides areexpressed as

    R.sub.VR =cos (δ)                                    (4)

    R.sub.VL ≈cos (δ)                            (5)

where δ is the front-wheel steering angle (obtainable by dividing thesteering wheel angle by the steering gear ratio). Therefore, thefront-rear wheel velocity ratio R_(V) can be given by cos (δ) withoutregard to the right or left of the vehicle.

However, the equations (4) and (5) hold true only when the vehicle isrunning at a low velocity (more accurately, when the lateralacceleration G_(Y) is low). Accordingly, the computation of the velocityratio R_(V) based on the equations (4) and (5) is carried out only whenthe vehicle body velocity V_(BM) is low as shown by the followingequation.

When

    V.sub.BM <30 km/h, R.sub.V =cos (δ)                  (6)

When the vehicle body velocity V_(BM) is relatively high, the velocityratio R_(V) is set to a constant value by the following equation.

When

    ≧30 km/h, R.sub.V =1                                (7)

V_(BM) is the vehicle body velocity V_(B) computed from the executionresult of the preceding cycle of the main routine. The computation ofthe vehicle body velocity V_(B) will be described later.

In the case where the vehicle is a front-engine front-drive (FF)vehicle, when the vehicle turns while not being braked, the referencewheel velocity V_(S) follows the wheel velocity of the outside rearwheel. In this case, the gravity-center velocity of the vehicle iscomputed by adding a correction to the reference wheel velocity V_(S),the correction being based on 1/2 of average inside-outside wheelvelocity difference ΔV_(IA) and a velocity difference between the rearaxle velocity and the gravity-center velocity. Since such computation ofgravity-center velocity is complicated, it is assumed that thegravity-center velocity is equal to an intermediate value between thevelocities in the front axle position and gravity-center position.Thereupon, an unfiltered gravity-center velocity V_(CGO) can be computedby the following equation.

    V.sub.CGO =(V.sub.S -ΔV.sub.IA /2)×(1+(1/R.sub.V))/2(8)

On the other hand, when the vehicle turns while being braked, it isthought that the reference wheel velocity V_(S) follows the outsidefront wheel velocity. In this case, the unfiltered gravity-centervelocity V_(CGO) is computed by adding a correction to the referencewheel velocity V_(S), the correction being based on 1/2 of averageinside-outside wheel velocity difference ΔV_(IA) and a velocitydifference between the front axle velocity and the gravity-centervelocity. That is, the gravity-center velocity V_(CGO) can be obtainedfrom the following equation.

    V.sub.CGO =(V.sub.S -ΔV.sub.IA /2)×(1+R.sub.V)/2(9)

Subsequently, the gravity-center velocity V_(CGO) is continuouslyfiltered twice (f_(c) =6 Hz), so that a filtered gravity-center velocityV_(CG) (=LPF(LPF(V_(CGO))) is obtained.

In computing the gravity-center velocity V_(CG) of the vehicle, whetherthe vehicle is braked or not is determined based on the brake flagF_(b).

Usually, since the gravity-center velocity V_(CG) follows the vehiclebody velocity V_(B), the gravity-center velocity V_(CG) is set as thevehicle body velocity V_(B). That is, the vehicle body velocity V_(B) isusually computed by the following equation.

    V.sub.B =V.sub.CG                                          (10)

However, in a situation such that the reference wheel having thereference wheel velocity V_(S) is locked and the brake pressure controlby an anti-skid brake system (ABS) is started for the reference wheel,the reference wheel velocity V_(S) is reduced by following the slip ofthe reference wheel. That is, the reference wheel velocity V_(S) issubstantially reduced as compared with the actual vehicle body velocity.

In such a situation, the ECU 23 determines whether or not apredetermined separation condition based on the longitudinal G_(X) hasbeen satisfied. If the separation condition is satisfied, the followingof the vehicle body velocity V_(B) after the gravity-center velocityV_(CG) is stopped, and the vehicle body velocity V_(B) separates fromthe gravity-center velocity V_(CG). After this separation, in the ECU23, the vehicle body velocity V_(B) is estimated by assuming that thevehicle body velocity V_(B) is reduced at a predetermined gradient.

More specifically, the separation condition is such that when a timedifferential of the gravity-center velocity V_(CG) and a separationdetermination value are denoted by ΔV_(CG) and G_(XS), respectively, thevehicle body velocity V_(B) is separated from the gravity-centervelocity V_(CG) when a condition of ΔV_(CG) ≦G_(XS) continues for 50msec or a condition of ΔV_(CG) ≦-1.4 g (g is a gravitationalacceleration) is satisfied. Here, the separation determination valueG_(XS) is set by the following equation.

    G.sub.XS =-(|G.sub.X |+0.2)

provided that

    -1.4 g≦G.sub.XS ≦-0.35 g                     (11)

If the above separation condition is satisfied, the vehicle bodyvelocity V_(B) is estimated by the following equation.

    V.sub.B =V.sub.BM -ΔG                                (12)

V_(BM) denotes a vehicle body velocity before the separation conditionis satisfied, and ΔG denotes a gradient set by the following equation.

    ΔG=(|G.sub.X +0.15)

provided that

    -1.2 g≦ΔG≦-0.3 g                       (13)

In the ECU 23, when the vehicle body velocity V_(B) is estimated whilebeing separated from the gravity-center velocity V_(CG), a separationend condition, in which the vehicle body velocity V_(B) can follow thegravity-center velocity V_(CG) again, is expressed by the followingequation.

    V.sub.CG >V.sub.BM                                         (14)

C: Slip factor of vehicle

Next, in the ECU 23, a correction based on the aforementioned averagevelocity difference ΔV_(IA) and the velocity ratio R_(V) is made to thevehicle body velocity V_(B), and a reference wheel velocity V_(R) (i) ateach wheel position is computed. That is, the reference wheel velocityV_(R) (i) is computed by the following equation.

    V.sub.R (i)-V.sub.B× 2/(1+R.sub.V)+(or -)V.sub.IA /2 (15)

Explaining the positive/negative sign of the second term of the equation(15), when the vehicle turns clockwise, (+) is used for the computationof the outside reference wheel velocity, and (-) is used for thecomputation of the inside reference wheel velocity. On the other hand,when the vehicle turns counterclockwise, the use of the positive andnegative signs is reverse.

The slip factor S_(R) (i) for each wheel is determined by filtering(f_(c) =10 Hz) by the equation (17) after the computation by theequation (16) is made.

    S.sub.R0 (i)=(V.sub.R (i)-V.sub.W (i))/V.sub.R (i)         (16)

    S.sub.R (i)=LPF(S.sub.R0 (i))                              (17)

S_(R0) (i) denotes an unfiltered slip factor.

D: Gravity-center slip angular velocity

If the angular velocity of vehicle around the center of turn during theturning of the vehicle (velocity of vehicle revolution) is ω, therelation between the gravity-center slip angular velocity dβ and the yawrate γ is expressed as

    γ=dβ(=βg)+ω                          (18)

where βg is a gravity-center slip angle.

If the gravity-center slip angle βg is small, there is a relationexpressed by the following equation between the vehicle body velocityV_(B) and the vehicle velocity V.

    V.sub.B =V×cos(βg)=V                            (19)

Also, there is a relation expressed by the following equation betweenthe vehicle velocity V and the lateral G_(Y).

    G.sub.Y =V×ω                                   (20)

Eliminating ω and V from the above three equations (18), (19), and (20),an unfiltered gravity-center slip angular velocity dβ₀ can be determinedfrom the following equation.

    dβ.sub.0 =γ-G.sub.Y /V.sub.B                    (21)

Therefore, the ECU 23 computes the unfiltered gravity-center slipangular velocity dβ₀ based on the above equation (21).

Subsequently, in the ECU 23, the gravity-center slip angular velocity dβis determined by filtering (f_(c) =2 Hz) the gravity-center slip angularvelocity dβ₀ as follows:

    dβ=LPF (dβ.sub.0)                                (22)

In order to make the sign of the gravity-center slip angular velocity dβpositive on the understeer (US) side and negative on the oversteer (OS)side, without regard to the vehicle turn direction, the computed slipangular velocity dβ is multiplied by (-) to be inverted in sign when thevehicle turns clockwise.

If a condition of V_(B) <10 km/h is met when the vehicle runs at a lowvelocity, in the ECU 23, the computation of the gravity-center slipangular velocity dβ is inhibited to prevent the overflowing ofcomputations, and the value of the gravity-center slip angular velocitydβ is set at 0.

Condition of manipulations

E: Steering wheel angular velocity

It is assumed that a steering wheel angle θ is changed as shown in FIG.5. A steering wheel angular velocity θ_(A) when the steering wheel angleθ is changed can be obtained by dividing the variation of the steeringwheel angle θ by the time required for the change. If time n is areference, and the steering wheel angle θ is changed by Δθ(n+4) at timen+4, as shown in FIG. 5, for example, in the ECU 23, a steering wheelangular velocity θ_(A0) (n+4) at time n+4 is calculated as follows:

    θ.sub.A0 (n+4)=Δθ(n+4)/(4×T)       (23)

where T is the control period for the aforementioned main routine.

When the steering wheel angle θ is not changed, it is assumed that theangle θ is changed by a minimum variation Δθ_(MIN) in the same directionfor its last change. In this case, the steering wheel angular velocityθ_(A0) is obtained by dividing the minimum variation Δθ_(MIN) by aperiod of time required for its change. For example, in the ECU 23, asteering wheel angular velocity θ_(A0) (n+2) at time n+2 is computed asfollows:

    θ.sub.A0 (n+2)=Δθ.sub.MIN /(2×T)   (24)

Then, the steering wheel angular velocity θ_(A0) is filtered (f_(c) =2Hz), whereupon a filtered steering wheel angular velocity θ_(A) isobtained as follows:

    θ.sub.A =LPF (θ.sub.A0)                        (25)

F: Effective steering wheel angular velocity value

In the ECU 23, the absolute value of the steering wheel angular velocityθ_(A) is filtered, and an effective steering wheel angular velocityvalue θ_(AE) is calculated as follows:

    θ.sub.AE =LPF (|θ.sub.A |)   (26)

The value of the cutoff frequency f_(c) for filtering process differsdepending on whether the steering wheel angle θ tends to increase ordecrease, that is, the positive or negative value of the steering wheelangular velocity θ_(A). For example, when the steering wheel angularvelocity θ_(A) takes a positive value, f_(c) is set at 20 Hz. On theother hand, when the steering wheel angular velocity θ_(A) takes anegative value, f_(c) is set at 0.32 Hz.

G: Pedal stroke velocity of brake pedal

In the ECU 23, a pedal stroke velocity V_(ST) is obtained by filtering(f_(c) =1 Hz) finite differences in the pedal stroke S_(t), that is, thetime differential thereof as follows:

    V.sub.ST =LPF (S.sub.t (n)-S.sub.t (n-1))                  (27)

where S_(t) (n-1) is a pedal stroke obtained during the execution of thepreceding main routine, and S_(t) (n) is a pedal stroke obtained duringthe execution of the present main routine.

H: Brake flag for brake pedal

In the ECU 23, the aforementioned brake flag F_(b) is set in accordancewith the pedal stroke S_(t) and the pedal stroke velocity V_(ST) asfollows:

If the condition of S_(t) >S_(te) or V_(ST) >50 mm/s is met, F_(b) =1.If the above condition is not met, F_(b) =0.

S_(te) is a depth of depression of the brake pedal 3 when the pressurein the master cylinder 2 rises actually.

The brake flag F_(b) is used in selecting the reference wheel velocityV_(S) or computing the gravity-center velocity V_(CG).

I: Augmented depression flag for brake pedal

In the ECU 23, an augmented depression flag F_(PP) is set in accordancewith the pedal stroke velocity V_(ST) as follows:

If V_(ST) >50 mm/s, F_(PP) =1.

If V_(ST) <20 mm/s, F_(PP) =0.

The setting routine for the aforementioned augmented depression flagF_(PP) is shown in FIG. 6. In this setting routine, the pedal strokevelocity V_(ST) is read (Step S201), and the augmented depression flagF_(PP) is set (Steps S203 and S205) in accordance with the results ofdetermination in Steps S202 and S204.

Turn determination

Next, in the ECU 23, Step 3 (see FIG. 4), that is, the turndetermination of the vehicle is executed. In FIG. 3, the determinationof turn direction is executed in the operation block 38, and the detailsthereof is shown in FIG. 7. Also, the details of Step S3 is shown in theflowchart in FIG. 8.

As seen from FIG. 7, in the turn determination of the vehicle, thevehicle turn direction is determined in accordance with the steeringwheel angle θ and the yaw rate γ, and whether or not the steering wheelmanipulation by the driver is countersteer is determined.

First, in the ECU 23, a direction flag F_(ds) based on the steeringwheel angle θ is determined on the basis of the steering wheel angle θaccording to a map Mθ shown in FIG. 7. Specifically, when the steeringwheel angle θ exceeds 10 deg in the positive direction, the directionflag F_(ds) is set at 1. In this case, the direction flag F_(ds) (-1)indicates a clockwise turn of the vehicle. On the other hand, when thesteering wheel angle θ exceeds -10 deg in the negative direction, thedirection flag F_(ds) is set at 0, which is indicative of acounterclockwise turn of the vehicle. When the steering wheel angle θ isin a range of -10 deg≦θ≦10 deg, the direction flag F_(ds) is kept at thevalue set in the preceding determination routine (FIG. 8).

The above-described setting procedure for the direction flag F_(ds) isshown in Steps S301 to S304 in the flowchart shown in FIG. 8.

On the other hand, in the ECU 23, a direction flag F_(dy) based on theyaw rate γ is determined on the basis of the yaw rate γ according to amap Mγ shown in FIG. 7. Specifically, when the yaw rate γ exceeds 2 degin the positive direction, the direction flag F_(dy) is set at 1. Inthis case, the direction flag F_(dy) (=1) indicates a clockwise turn ofthe vehicle. On the other hand, when the yaw rate γ exceeds -2 deg inthe negative direction, the direction flag F_(dy) is set at 0, which isindicative of a counterclockwise turn of the vehicle. When the yaw rateγ is in a range of -2 deg≦θ≦2 deg, the direction flag F_(dy) is kept atthe value set in the preceding determination routine (FIG. 8).

The setting procedure for the direction flag F_(dy) is shown in StepsS305 to S308 in the flowchart shown in FIG. 8.

As shown in FIG. 7, the direction flags F_(ds) and F_(dy) are suppliedto a switch SW_(F), and this switch SW_(F) is shifted in response to aswitching signal delivered from a determination section 40. Therefore,in the ECU 23, the direction flag outputted from the switch SW_(F) isselected as a turn flag F_(d).

When the brake pressure of at least one front wheel is under the ABScontrol and the brake flag F_(b) is set at 1, the determination section40 delivers a switching signal for shifting the switch SW_(F) to anupper position as indicated by the arrow mark of broken line in FIG. 7.In this case, the direction flag F_(ds) based on the steering wheelangle θ is set as the turn flag F_(d) as follows.

    F.sub.d =F.sub.ds

However, when the aforesaid condition is not met, the switching signalis not delivered from the determination section 40. In this case, theswitch SW_(F) is at a switching position indicated by the arrow mark ofsolid line. The direction flag F_(dy) based on the yaw rate γ is set asthe turn flag F_(d) as follows.

    F.sub.d =F.sub.dy

The setting procedure for the turn flag F_(d) is shown in Steps S309 toS311 in the flowchart shown in FIG. 8.

Subsequently, in the ECU 23, it is determined whether or not thesteering wheel manipulation by the driver is countersteer. That is, inStep S312 in the flowchart shown in FIG. 8, it is determined whether ornot the values of the direction flags F_(ds) and F_(dy) are not equal toeach other. If the result of determination in this step is Yes, that is,if the direction of yawing acting on the vehicle does not agree with theoperation direction of the steering wheel, 1 is set in a countersteerflag F_(cs) (Step S314). If the result of determination in Step S312 isNo, on the other hand, 0 is set in a countersteer flag F_(cs) (StepS315).

Computation of target yaw rate

Next, in Step S4, that is, in the operation block 39 in FIG. 3, the ECU23 computes the target yaw rate γ_(t) of the vehicle. The details of theoperation block 39 is shown in FIG. 9.

As seen from FIG. 9, the vehicle body velocity V_(B) and the front-wheelsteering angle δ are supplied to the operation section 42, where asteady-state gain is determined. Subsequently, the steady-state gain isfiltered in sequence in the following filter sections 44 and 46. As aresult, the target yaw rate γ_(t) is determined.

When the aforesaid steering gear ratio is denoted by ρ, the front-wheelsteering angle δ is determined by the following equation.

    δ=θ/ρ                                      (28)

The steady-state gain is a value indicative of the response of yaw rateacting on the vehicle to the operation of steering wheel. Specifically,the steady-state gain can be derived from a linear two-wheel model ofthe vehicle. In the first-stage filter section 44, a low-pass filter(LPF1) for noise removal is used, while in the second-stage filtersection 46, a low-pass filter (LPF2) for the first-order-delay response.

Therefore, in the ECU 23, the target yaw rate γ_(t) is computed asfollows:

    γ.sub.t =LPF2((LPF1(V.sub.B /(1+A×V.sub.B.sup.2)×(δ/L))             (29)

where A and L are a stability factor and a wheel base, respectively.

Computation of required yaw moment

Next, in Step S5 (FIG. 4), that is, in the operation block 41 in FIG. 3,ECU 23 computes a required yaw moment γ_(d). The details of theoperation block 41 and Step S5 are shown in FIG. 10 and FIG. 11,respectively.

As seen from FIG. 10, the operation block 41 has a subtractor section48. In this subtractor section 48, the difference between the target yawrate γ_(t) and the yaw rate γ, that is, a yaw rate deviation Δγ iscomputed. The computation procedure for the yaw rate deviation Δγ isshown in Steps S501 and S502 in the flowchart shown in FIG. 11.

Step S502 will be described in detail. The sign of the yaw ratedeviation Δγ is inverted so that it is positive on the understeer (US)side and negative on the oversteer (OS) side when the vehicle turnscounterclockwise. The vehicle turn direction is determined by the valueof the aforesaid turn flag F_(d).

Further in Step S502, a maximum yaw rate deviation Δγ_(MAX) is computedaccording to the following equation by filtering the absolute value ofthe yaw rate deviation Δγ.

    Δγ.sub.MAX LPF(|Δγ|)(30)

The cutoff frequency f_(c) used in this filtering varies depending onwhether the yaw rate deviation Δγ is increased or decreased. Forexample, when the yaw rate deviation Δγ is increased, f_(c) is set at 10Hz, while when the yaw rate deviation Δγ is decreased, f_(c) is set at0.08 Hz.

When the yaw moment control, mentioned later, is finished (or when thevalue of a yaw moment control on-off flag F_(ym) is 0), the absolutevalue of the yaw rate deviation Δγ is given to the maximum yaw ratedeviation Δγ_(MAX) as follows:

    Δγ.sub..sub.MAX =|Δγ|(31)

Next, the yaw rate deviation Δγ is supplied to a differentiator section50 (FIG. 10). In the differentiator section 50, the finite difference ofthe yaw rate deviation Δγ, that is, a yaw rate deviation derivativeΔγ_(s) is computed. Subsequently, the derivative Δγ_(s) is filtered(f_(c) =5 Hz). That is, in the ECU 23, the yaw rate deviation derivativeΔγ_(s) is computed as follows:

    Δγ.sub.s =LPF(Δγ-Δγ.sub..sub.m)(32)

In the equation (32), Δγ_(m) is a yaw rate deviation computed in thepreceding computation routine. As explained regarding the yaw ratedeviation Δγ, the sign of the yaw rate deviation derivative Δγ_(s) isinverted when the vehicle turns counterclockwise.

The computation of the yaw rate deviation derivative Δγ_(s) is carriedout in Step S503 in the flowchart shown in FIG. 11.

Thereafter, the yaw rate deviation derivative Δγ_(s) is supplied to amultiplier section 52, where the derivative Δγ_(s) is multiplied by aproportional gain K_(P), as shown in FIG. 10. Also, the yaw ratedeviation Δγ is supplied to a multiplier section 54, where the yaw ratedeviation Δγ is multiplied by an integral gain K_(i). The outputs fromthe multiplier sections 52 and 54 are added in an adder section 56.

Further, the output from the adder section 56 is supplied to amultiplier section 58. In this multiplier section 58, the output of theadder section 56 is multiplied by a correction value C_(pi), whereuponthe required yaw moment γ_(d) is computed. Therefore, in the ECU 23, therequired yaw moment γ_(d) is computed as follows:

    γ.sub.d =(Δγ.sub.s ×K.sub.p +Δγ.sub. ×K.sub.i)×C.sub.pi                            (33)

The correction value C_(pi) is set to a different value depending onwhether the vehicle is braked or not. For example, the correction valueC_(pi) is set as follows:

When the vehicle is braked (F_(b) =1), C_(pi) =1.0

When the vehicle is not braked (F_(b) =0), C_(pi) =1.5

The computation of the required yaw moment γ_(d) is carried out in StepsS504 and S505 in the flowchart shown in FIG. 11. In Step S504, theproportional and integral gains K_(p) and K_(i) are computed. Thedetails of the computation of the proportional gain K_(p) is shown inFIG. 12.

As seen from FIG. 12, in computing the proportional gain K_(p), the ECU23 has different reference values K_(p), (e.g., 4 kgm/s/(deg/s²)) andK_(po), (e.g., 5 kgm/s/(deg/s²)) depending on whether the vehicle turnson the understeer side or on the oversteer side. A switch Sw_(p) is usedfor the selection between the values K_(pu) and K_(po).

The switch Sw_(p) is shifted in response to a determination signaldelivered from a determination section 60. The determination section 60delivers a determination signal such that the switch Sw_(p) is shiftedto the side of the reference value K_(pu), when in the understeer modein which the yaw rate deviation derivative Δγ.sub._(s) is 0 or more.

The reference value outputted from the switch Sw_(p) is multipliedsuccessively by correction factors K_(p1), K_(p2) and K_(p3) inmultiplier sections 62, 64 and 66, respectively, whereby theproportional gain K_(p) is obtained.

Thus, the proportional gain K_(p) is computed as follows in accordancewith the turn characteristic of the vehicle.

    K.sub.p =K.sub.pu ×K.sub.p1 ×K.sub.p2 ×K.sub.p3 (understeer mode)

    K.sub.p =K.sub.po ×K.sub.p1 ×K.sub.p2 ×K.sub.p3 (oversteer mode)

If the vehicle body is subjected to the yaw moment control before thevehicle reaches its critical travel region, it will inevitably make thedriver feel uneasy. To avoid this, the proportional gain K_(p) iscorrected by the correction factor K_(p1) only when the yaw ratedeviation Δγ or the lateral G_(Y) of the vehicle body is large. As aresult, the proportional gain K_(p) functions effectively. Specifically,correction factor K_(p1) is computed according to the computationroutine shown in FIG. 13.

In the computation routine shown in FIG. 13, it is first determinedwhether or not the maximum yaw rate deviation Δγ_(MAX) exceeds 10 deg/s(Step S506). If the result of this determination is Yes, 1.0 is set inthe correction factor K_(p1) (Step S507).

If the result of the determination in Step S506 is No, the absolutevalue of the lateral G_(Y) acting on the vehicle body is filtered asfollows, and the average lateral G_(YA) is computed (Step S508).

    G.sub.YA LPF(|G.sub.Y |)

For the cutoff frequency f_(c) in this filtering, f_(c) is set at 20 Hzwhen the lateral G_(Y) tends to be increased, and at 0.23 Hz when thelateral G_(Y) tends to be decreased.

Thereafter, a reference lateral acceleration G_(YR) is computed inaccordance with the vehicle body velocity V_(B) (Step S509).Specifically, a map as shown in FIG. 14 has been stored in advance inthe memories of the ECU 23, and the reference lateral G_(YR) is readfrom this map based on the vehicle body velocity V_(B). Since the travelof the vehicle becomes unstable easily as the vehicle body velocityV_(B) increases, as seen from a map in FIG. 14, the reference lateralG_(YR) is decreased gradually with the increase in the vehicle bodyvelocity V_(B) in the high-speed region.

After the average lateral G_(YA) and the reference lateral G arecomputed as described above, it is determined whether or not the averagelateral G_(YA) is larger than the reference lateral G_(YR) (Step S510).If the result of this determination is Yes, 1.0 is set in the correctionfactor K_(p1) (Step S507). If the result of the determination in StepS510 is No, 0.05 is set in the correction factor K_(p1) (Step S511).

The correction factor K_(p2) is used to correct the proportional gainK_(p) for the following reason. If the actual yaw rate γ is made simplyto follow up the target yaw rate γ_(t) in the case where the frictioncoefficient of road surface is low, that is, the vehicle is running on alow-μ road, the lateral force acting on the vehicle body of (a) in FIG.15 immediately reaches its critical value, and the gravity-center slipangle β of the vehicle body increases suddenly, so that the vehicle bodyof (a) may possibly spin.

Therefore, if the proportional gain K_(p) is corrected by the correctionfactor K_(p2) set appropriately, it is believed that the gravity-centerslip angle β of the vehicle body is kept small, so that the vehicle bodycan be prevented from spinning, as shown in the vehicle in FIG. 15(b).FIG. 15 (c) shows a vehicle running on high-μ road.

Specifically, the correction factor K_(p2) is determined by the settingroutine shown in FIG. 16. In this setting routine, the gravity-centerslip angular velocity dβ is first read (Step S512), and then a referencecorrection factor K_(cd) is read from a map shown in FIG. 17 based onthe gravity-center slip angular velocity dβ (Step S513). As seen fromthe map in FIG. 17, for example, the reference correction factor K_(cd)has a characteristic of decreasing gradually from the maximum value(1.0) when the gravity-center slip angular velocity dβ becomes higherthan 2 deg/s and being kept at the minimum value (0.1) when thegravity-center slip angular velocity dβ becomes 5 deg/s and over.

In the next Step S514, the yaw rate deviation Δγ is read, and whether ornot the turn of the vehicle is in an understeer (US) mode is determinedaccording to the sign of the yaw rate deviation Δγ (Step S515). If theresult of this determination is Yes, the reference correction factorK_(cd) is set in the correction factor K_(p2) (Step S516). If the resultof the determination is No, 1.0 is set in the correction factor K_(p2)(Step S517). That is, when the turn of the vehicle is in an understeermode, the correction factor K_(p2) is set based on the gravity-centerslip angular velocity dβ. When the turn of the vehicle is in anoversteer mode, the correction factor K_(p2) is set at a constant 1.0.The subsequent steps from Step S518 in the flowchart shown in FIG. 16will be described later.

The correction factor K_(p3) is used to correct the proportional gainK_(p) for the following reason. When the vehicle is running on a roughroad, a vibration component is added to the output of the yaw ratesensor 30, that is, the yaw rate γ. The vibration component of the yawrate γ is amplified when the yaw rate deviation derivative Δγ_(s) iscomputed, so that the derivative Δγ_(s), that is, the required yawmoment γ_(d) is not computed accurately. As a result, erroneousoperation may occur in the control using the required yaw moment γ_(d),or the stability of the control may be impaired. Accordingly, thecorrection factor K_(p3) is used to decrease the proportional gain K_(p)in order to eliminate an effect of the vibration component on thederivative Δγ_(s).

To determine the correction factor K_(p3), a vibration component γ_(v)of yaw rate is first computed. As shown in the block diagram shown inFIG. 18, a yaw rate γ₀, which is the output from the yaw rate sensor 30,and a yaw rate γ_(0M) obtained in the preceding setting routine (FIG.19) are supplied to a subtractor section 68 (Step S522 in FIG. 19). Inthis subtractor section 68, a deviation between the yaw rate γ₀ and theyaw rate Y_(0M), that is, a derivative Δγ.sub.₀ is computed.

Subsequently, the derivative Δγ₀ is filtered (f_(c) =12 Hz) in a firstfilter section 69, and supplied to a subtractor section 70. The outputof the first filter section 69 is filtered (f_(c) =10 Hz) in a secondfilter section 71, and supplied to the subtractor section 70. In thesubtractor section 70, a deviation between two filtered derivatives Δγ₀is computed, and the deviation is outputted to an operation section 72.In this operation section 72, the absolute value of the derivativedeviation is determined, and this absolute value of the deviation isfiltered (f_(c) =0.23 Hz) in a third filter section 73. As a result, thevibration component γ_(v) of yaw rate is outputted from the third filtersection 73 (Step S523 in FIG. 19). Therefore, the vibration componentγ_(v) of yaw rate is computed by the following two equations.

    Δγ.sub..sub.0 =γ.sub.0 -γ.sub.0M   (34)

    γ.sub.v =LPF3(|LPF1(Δγ.sub..sub.0)-LPF2(Δγ.sub.0)|)                                                (35)

Next, as indicated in Step S524 in FIG. 19, the correction factor K_(p3)is computed based on the vibration component γ_(v) of yaw rate.Specifically, a map as shown in FIG. 20 has been stored in advance inthe memories of the ECU 23, and the correction factor K_(p3) is readbased on the vibration component γ_(v) of yaw rate. As seen from the mapin FIG. 20, for example, the correction factor K_(p3) has acharacteristic of decreasing suddenly from 1.0 with the increase in thevibration component γ_(v) when the vibration component γ_(v) of yaw ratebecomes larger than 10 deg/s, and being kept at a constant value of 0.2when the vibration component γ_(v) becomes 15 deg/s and over.

Next, the computation of the aforesaid integral gain K_(i) is shown in ablock diagram of FIG. 21. In this block diagram too, as in the case ofcomputation of the proportional gain K_(p), a reference integral gainK_(i0) (e.g., 10 kgm/s/(deg/s²)) is prepared in advance for use. Thereference integral gain K_(i0) is multiplied by a correction factorK_(i1) in a multiplier section 74, and the output of the multipliersection 74 is multiplied by a correction factor K_(i2) in the multipliersection 76. The output from this multiplier section 76 is the integralgain K_(i). Thus, the integral gain K_(i) is computed as follows:

    K.sub.i =K.sub.i0 ×K.sub.i1 ×K.sub.i2          (36)

The correction factor K_(i1) is used to reduce the integral gain K_(i)for the following reason. If the front-wheel steering angle increases,an error in the target yaw rate γ_(t), that is, an error in the yaw ratedeviation Δγ further enlarges, possibly entailing erroneous operation ofcontrol using the yaw rate deviation. In this situation, therefore, theintegral gain K_(i) is reduced by means of the correction factor K_(i0).

Specifically, the correction factor K_(i1) is set based on the steeringwheel angle θ from a map shown in FIG. 22. As seen from FIG. 22, thecorrection factor K_(i1) has a characteristic of decreasing suddenlyfrom the maximum value with the increase in the steering wheel angle θwhen the absolute value of the steering wheel angle θ is as large as 400deg and being kept at the minimum value of 0.5 when the steering wheelangle θ becomes 600 deg and over.

The correction factor K_(i2) is used to reduce the integral gain K_(i)for the same reason for the case of the correction factor K_(p2) for theproportional gain K_(p). Therefore, the computation procedure for thecorrection factor K_(i2) is shown together with the setting routine forthe correction factor K_(p2) in FIG. 16.

In Step S518 in FIG. 16, the yaw rate deviation derivative Δγ_(s) isread, and whether or not the turn of the vehicle is in an understeermode is determined according to the sign of the derivative Δγ_(s) (StepS519). If the result of this determination is Yes, the aforesaidreference correction factor K_(cd) (see FIG. 17) is set in thecorrection factor K_(i2) (Step S520). If the result of the determinationis No, 1.0, which is the maximum value, is set in the correction factorK_(i2).

Yaw moment control of vehicle

When the required yaw moment γ_(d) is computed in the aforementionedmanner, the yaw moment control is carried out in Step S6 in the mainroutine shown in FIG. 4, that is, in an operation block 78 in FIG. 3.The detail of the operation block 78 is shown in FIG. 23.

First, the operation block 78 in FIG. 23 has a determination section 80for determining the start or end of the yaw moment control. In thisdetermination section 80, an on-off flag F_(ymc) is settled inaccordance with the required yaw moment γ_(d).

Specifically, the on-off flag F_(ymc) is settled in a determinationcircuit shown in FIG. 24. This determination circuit includes an ORcircuit 81 having two input terminals, and on- and off-signalscorresponding to the required yaw moment γ_(d) are applied to the inputterminals of the OR circuit 81. More specifically, the on-signal isapplied to one input terminal of the OR circuit 81 if the required yawmoment γ_(d) is lower than a threshold value γ_(os) (e.g., -100 kgm/s)on the oversteer side. If the required yaw moment γ_(d) is higher thananother threshold value γ_(us) (e.g., 200 kgm/s) on the understeer side,on the other hand, the on-signal is applied to the other input terminalof the OR circuit 81. Thus, if either of the threshold values areexceeded by the required yaw moment γ_(d), the on-signal is deliveredfrom the output terminal of the OR circuit 81, and is applied to the setterminal S of a flip-flop 82. Inconsequence, the on-off flag F_(ymc),i.e. on-off flag F_(ymc) (=1) indicative of the start of control in thiscase, is outputted from the output terminal Q of the flip-flop 82.

The absolute value (100 kgm/s) of the threshold value γ_(os) on theoversteer side is smaller than the absolute value (200 kgm/s) of thethreshold value γ_(us) on the understeer side. Thus, the output timingfor the on-off flag F_(ymc) (-1) is earlier on the oversteer side thanthe understeer side. That is, the start timing for the yaw momentcontrol is earlier on the oversteer side than on the understeer side asdescribed later.

The reset terminal R of the flip-flop 82 can be supplied with a resetsignal, which settles the reset timing for the on-off flag F_(ymc), thatis, the timing for the delivery of the flag F_(ymc) =0 from theflip-flop 82.

As shown in FIG. 24, a circuit for generating the reset signal includesa switch 83, which has two input terminals. A first determination timet_(ST1) (e.g., 152 msec) is supplied to one input terminal of the switch83, and a second determination time t_(ST2) (e.g., 504 msec) to theother input terminal.

The switch 83 can be shifted in response to a switching signal deliveredfrom a determination section 84. When the behavior of the vehicle bodyis stable, that is, if all the following conditions are fulfilled, thedetermination section 84 delivers a switching signal to the switch 83 sothat the switch 83 is shifted to output the first determination timet_(ST1) as an end determination time t_(ST). However, if any of theseconditions is not fulfilled, the second determination time t_(ST2) isoutputted as the end determination time t_(ST).

Condition 1: target yaw rate γ_(t) <10 deg/s

Condition 2: yaw rate γ<10 deg/s

Condition 3: effective steering wheel angular velocity value θ_(AE) <200deg/s

Next, the end determination time t_(ST) is supplied to a determinationsection 85. In this determination section 85, it is indicated that abrake pressure control signal is in a held state or a noncontrolledstate (control mode M(i), mentioned later, is in a hold mode ornoncontrol mode) for the control of brake pressure of each wheel, and itis determined whether the state continues for the end determination timet_(ST) and longer. If the result of this determination is Yes, thedetermination section 85 outputs an end indication flag F_(ST) (i)=1. Ifthe result is No, the determination section 85 outputs an end indicationflag F_(ST) (i)=0. Character i suffixed to each end indication flagF_(ST) represents the aforesaid wheel number. The control signal forcontrolling the brake pressure for each wheel will be mentioned later.

The end indication flags F_(ST) (i) are supplied individually to theinput terminals of an AND circuit 86. The output terminal of the ANDcircuit 86 is connected to one input terminal of an OR circuit 87. Theother input terminal of the OR circuit 87 is supplied with an on-signalwhen the vehicle body velocity V_(B) is lower than 10 km/h. The outputterminal of the OR circuit 87 is connected to the reset terminal R ofthe flip-flop 82.

The AND circuit 86 supplies the on-signal to the OR circuit 87 when allinput signals are on, that Is, all values of the end indication flagF_(ST) (i) are 1. The OR circuit 87 supplies an on-signal to the resetterminal R of the flip-flop 82 when one of the input signal thereto isthe on-signal. Thus, the reset signal is supplied to the flip-flop 82 ifthe vehicle body velocity V_(B) is lower than 10 km/h or if the brakepressure control signal fulfills the aforesaid conditions for any of thefour wheels.

When the reset signal is given to the flip-flop 82, the flip-flop 82outputs the on-off flag F_(ymc) (=0), which is indicative of thetermination of the-control.

As shown in FIG. 23, the determination section 80 supplies the on-offflag F_(ymc) to a brake pressure control mode determination section 88.In this determination section 88, the brake pressure control mode foreach wheel is selected in accordance with the required yaw moment γ_(d)and the turn flag F_(d) in the case where the value of the suppliedon-off flag F_(ymc) is 1.

Specifically, brake pressure control execution flags F_(cus) and F_(cos)are first set from the map shown in FIG. 25 based on the required yawmoment γ_(d) compared with its threshold values. The control executionflag F_(cus) is a flag in the understeer mode of vehicle turn, and thecontrol execution flag F_(cos) is a flag in the oversteer mode ofvehicle turn.

Understeer mode:

    F.sub.cus =1 when γ.sub.d >γ.sub.dUS1 (=100 kgm/s)

    F.sub.cus =0 when γ.sub.d <γ.sub.dUS0 (=80 kgm/s)

Oversteer mode:

    F.sub.cos =1 when γ.sub.d <γ.sub.dOS1 (=-80 kgm/s)

    F.sub.cos =0 when γ.sub.d >γ.sub.dOS0 (=-60 kgm/s)

Next, the brake pressure control modes M(i) for the individual wheelsare selected in accordance with the combination of the turn flag F_(d)and the control execution flags F_(cus) and F_(cos). FIG. 26 shows aselection routine for these control modes.

In the control mode selection routine of FIG. 26, it is first determinedwhether or not the value of the turn flag F_(d) is 1 (Step S601). If theresult of this determination is Yes, that is, if it is concluded thatthe vehicle is turning clockwise, it is determined whether or not thevalue of the control execution flag F_(cus) is 1 (Step S602). If theresult of determination in Step S602 is Yes, then the vehicle turningclockwise has a marked tendency to understeer, and the required yawmoment γ_(d) is at a great value greater than the threshold valueγ_(dUS1), so that the vehicle requires a turning moment. In the nextStep S603, therefore, the control mode M(1) for the front-left wheelFW_(L) is adjusted to the reduce-pressure mode, the control mode M(4)for the rear-right wheel RW_(R) to the intensify-pressure mode, and thecontrol modes M(2) and M(3) for the front-right and rear-left wheelsFW_(R) and RW_(L) to the noncontrol mode.

If the result of determination in Step S602 is No, it is determinedwhether or not the value of the control execution flag F_(cos) is 1(Step S604). If the result of this determination is Yes, then thevehicle turning clockwise has a marked tendency to oversteer, and therequired yaw moment γ_(d) is at a great value greater than the thresholdvalue γ_(dOS1) on the negative side, so that the vehicle requires arestoration moment. In the next Step S605, therefore, the control modeM(1) for the front-left wheel FW_(L) is adjusted to theintensify-pressure mode, the control mode M(4) for the rear-right wheelRW_(R) to the reduce-pressure mode, and the control modes M(2) and M(3)for the front-right and rear-left wheels FW_(R) and RW_(L) to thenoncontrol mode.

If both the results of determinations in Steps S602 and S604 are No, theturning vehicle has no marked tendency either to understeer or tooversteer. In this case, the control modes M(1) and M(4) for thefront-left and rear-right wheels FW_(L) and RW_(R) are both adjusted tothe hold mode, and the control modes M(2) and M(3) for the front-rightand rear-left wheels FW_(R) and RW_(L) to the noncontrol mode (StepS606).

If the result of determination in Step S601 is No, that is, if it isconcluded that the vehicle is turning counterclockwise, on the otherhand, it is determined whether or not the value of the control executionflag F_(cus) is 1 (Step S607).

If the result of determination in Step S607 is Yes, the vehicle requiresa turning moment, as in the case of the aforementioned clockwise turn.In the next Step S608, therefore, in contrast with the case of theclockwise turn, the control mode M(2) for the front-right wheel FW_(R)is adjusted to the reduce-pressure mode, the control mode M(3) for therear-left wheel RW_(L) to the intensify-pressure mode, and the controlmodes M(1) and M(4) for the front-left and rear-right wheels FW_(L) andRW_(R) to the noncontrol mode.

If the result of determination in Step S607 is No, it is determinedwhether or not the value of the control execution flag F_(cos) is 1(Step S609). If the result of this determination is Yes, the vehiclerequires a restoration moment. In the next Step S610, therefore, thecontrol mode M(2) for the front-right wheel FW_(R) is adjusted to theintensify-pressure mode, the control mode M(3) for the rear-left wheelRW_(L) to the reduce-pressure mode, and the control modes M(1) and M(4)for the front-left and rear-right wheels FW_(L) and RW_(R) to thenoncontrol mode.

If both the results of determinations in Steps S607 and S609 are No, thecontrol modes M(2) and M(3) for the front-right and rear-left wheelsFW_(R) and RW_(L) are both adjusted to the hold mode, and the controlmodes M(1) and M(4) for the front-left and rear-right wheels FW_(L) andRW_(R) to the noncontrol mode, as in the case of the aforementionedclockwise turn (Step S611).

Table 1 below collectively shows the control modes M(i) described above.

                  TABLE 1                                                         ______________________________________                                        clockwise turn (Fd = 1)                                                                          counterclockwise turn (Fd = 0)                             F.sub.cus                                                                          1        0        0     1      0      0                                  F.sub.cos                                                                          0        1        0     0      1      0                                  ______________________________________                                        FW.sub.L :                                                                         reduce   intensify                                                                              hold  non-   non-   non-                               M(1)                         control                                                                              control                                                                              control                            FW.sub.R :                                                                         non-     non-     non-  reduce intensify                                                                            hold                               M(2) control  control  control                                                RW.sub.L :                                                                         non-     non-     non-  intensify                                                                            reduce hold                               M(3) control  control  control                                                RW.sub.R :                                                                         intensify                                                                              reduce   hold  non-   non-   non-                               M(4)                         control                                                                              control                                                                              control                            ______________________________________                                    

The control modes M(i) for the individual wheels and the required yawmoment γ_(d) selected in the determination section 88 are supplied to avalve control signal computing section 89, where control signals for thesolenoid valve units (inlet and outlet valves 12 and 13) for controllingthe respective brake pressures for the individual wheels are computed inaccordance with the control modes M(i) and the required yaw momentγ_(d).

Specifically, in the computing section 89, the control rate inincreasing or decreasing the brake pressure for each wheel is computedto obtain the required yaw moment γ_(d). In order to change the wheelbrake pressure by a fixed pressure value ΔP (e.g., ±5 kg/cm²) at a timein accordance with the control rate, a driving pulse, that is, a valvecontrol signal of the inlet or outlet valve 12 or 13 required for makinga change by the pressure value ΔP is computed. The valve control signalis represented by the pulse periods T_(PLS) and pulse widths W_(PLS)(i). In order to secure good response of brake pressure control, thepressure value ΔP in the initial cycle is set at ±10 kg/cm².

Referring to FIG. 27, there is shown the way the brake pressure for thewheel, that is, in the wheel cylinder is increased or decreased withevery pressure value ΔP.

The inlet and outlet valves 12 and 13 are supplied with the valvecontrol signals as based on the hold mode, and actuated in accordancewith the valve control signals. Since the actuation of the inlet andoutlet valves 12 and 13 is ordered with every control period T (8 msec)for the main routine, an actuation mode M_(PLS) (i) is set so thatactual valve actuation is carried out with every pulse period T_(PLS).

The following is a detailed description of the pulse period T_(PLS),pulse width W_(PLS) (i), and actuation mode M_(PLS) (i).

First, if the lateral force on the vehicle body is ignored, a variationΔM_(z) of the yaw moment acting on the vehicle body when the brakepressure for a front wheel (in the wheel cylinder) is changed by ΔP_(WC)can be expressed as follows:

    ΔM.sub.Z =ΔP.sub.WC ×B.sub.F ×TF/2 (37)

where B_(F) and T_(F) are the front brake coefficient (kg/cm² →kg) andfront tread, respectively, of the vehicle.

Accordingly, the control rate R_(PWC) (kg/cm² /s) of the brake pressureobtained when the required yaw moment γ_(d) is given can be expressed asfollows:

    R.sub.PWC =2×γ.sub.d /B.sub.F /T.sub.F         (38)

If the pressure value ΔP (5 or 10 kg/cm²) is fixed, the relationshipbetween the control rate R_(PWC) and the pulse period T_(PLS) leads tothe following equation:

    |R.sub.PWC |=ΔP/(T.sub.PLS ×T(=8 msec))(39)

Based on these two equations (38) and (39), the pulse period T_(PLS) isexpressed as follows:

    T.sub.PLS =ΔP×B.sub.F ×T.sub.F /(2×T×|γ.sub.d |)     (40)

where 2≦T_(PLS) <12 is given.

The pulse period T_(PLS) is also applied to the inlet and outlet valves12 and 13 of the solenoid valve unit on the rear-wheel side.

The pulse width W_(PLS) (i) is previously set in an experiment.According to this experiment, the master cylinder pressure and wheelbrake pressure (brake pressure) are adjusted to their respectivereference values. In this state, the time for the change of the wheelbrake pressure by the pressure value ΔP (5 or 10 kg/cm²) after theactuation of the inlet or outlet valve is measured. The pulse widthW_(PLS) (i) is set on the basis of the measured time. Since the pressureof discharge from the pump 16 (or 17) is utilized for the increase ofthe wheel brake pressure, the pulse width W_(PLS) (i) is set inconsideration of a delay in response of the pump 16 (or 17).

The actuation mode M_(PLS) (i) is set in accordance with the settingroutine shown in FIG. 28. In this setting routine, the control mode M(i)is determined first (Step S612). If the control mode M(i) is thenoncontrol mode, the values of an adding counter CNT_(I) (i) forintensify-pressure control and an adding counter CNT_(D) (i) forreduce-pressure control are both reset at 0, whereupon the actuationmode M_(PLS) (i) is set to the noncontrol mode (Step S613).

If the control mode M(i) is the pressure-hold mode, the hold mode is setfor the actuation mode M_(PLS) (i) (Step S614).

If the control mode M(i) is the intensify-pressure mode, the actuationof only the adding counter CNT_(I) (i) is started (Step S615). Then, itis determined whether or not the value of the adding counter CNT_(I) (i)has reached the pulse period T_(PLS) (Step S616). Immediately after theactuation of the adding counter CNT_(I) (i) is started, the result ofdetermination in Step S616 is No, and it is determined in Step S617whether or not the value of the adding counter CNT_(I) (i) is 0. In thiscase, the result of this determination is Yes. Accordingly, theintensify-pressure mode is set for the actuation mode M_(PLS) (i) (StepS618).

Thereafter, as the setting routine is executed repeatedly, the value ofthe adding counter CNT_(I) (i) is increased one by one. As long as theresult of determination in Step S616 remains No, the result ofdetermination in Step S617 is No, and the pressure-hold mode is set forthe actuation mode M_(PLS) (i) (Step S619).

If the result of determination in Step S616 becomes Yes with the passageof time, however, the value of the adding counter CNT_(I) (i) is resetat 0 (Step S620). In this case, the result of determination in Step S617becomes Yes, whereupon the intensify-pressure mode is set for theactuation mode M_(PLS) (i) (Step S618). As a result, as long as thecontrol mode M(i) is kept at the intensify-pressure mode, the actuationmode M_(PLS) (i) is set to the intensify-pressure mode with every pulseperiod T_(PLS).

If the control mode M(i) is the reduce-pressure mode, on the other hand,Steps S621 to S626 in the flowchart shown in FIG. 28 are executed,whereupon the actuation mode M_(PLS) (i) is set to the reduce-pressuremode with every pulse period T_(PLS).

Subsequently, in the next stage, an inhibitory section 90 (see FIG. 23)corrects the pulse width W_(PLS) (i) to inhibit the control of brakepressure when the operation of steering wheel is in a countersteer mode,when the wheel slip is excessive, or when the required yaw moment tendsto decrease. The inhibitory section 90 is shown in detail in the blockdiagram of FIG. 29.

The inhibitory section 90 includes three switches 91, 92 and 93. Thepulse width W_(PLS) (i) supplied from the computing section 89 in thepreceding stage is outputted as a pulse width W_(PLS1) (i) after passingthrough the switches 91, 92 and 93. The switches 91, 92 and 93 areshifted based on the values of flags set in setting sections 94, 95 and96. That is, when the switches 91, 92 and 93 are at switching positionsshown in the figure, the pulse width W_(PLS1) (i) is W_(PLS) (i). On theother hand, when any one of the switches 91, 92 and 93 is shifted fromthe position shown in the figure, the value of W_(PLS1) (i) is reset at0. Instead of resetting the pulse width W_(PLS1) (i) at 0, a valuesmaller than W_(PLS) (i) can be given to the pulse width W_(PLS1) (i).As seen from FIG. 29, the actuation mode M_(PLS) (i) passes through theinhibitory section 90 without change.

In the setting section 94, an inhibiting flag F_(K1) (i) forcountersteer is set. Specifically, the setting section 94 includes anAND circuit 97, the output of which is supplied to the switch 91 as theinhibiting flag F_(K1) (i). If all of the three input conditions of theAND circuit 97 are met, that is, if all the inputs are on, the value ofthe inhibiting flag F_(K1) (i) is set at 1. If any of the inputconditions is No, the value of the inhibiting flag F_(K1) (i) is set at0. The first input condition is on when the target wheel is a rearwheel, that is, when the wheel number i is 3 or 4, and the second inputcondition is on when the value of a countersteer flag F_(CS) is 1. Thethird input condition is on when the control mode M(i) is theintensify-pressure mode.

When, the value of the inhibiting flag F_(K1) (i) is 1, the switch 91 isshifted from the position shown in FIG. 29, whereupon the value of thepulse width W_(PLS1) (i) becomes 0.

FIG. 30 shows a setting routine for the inhibiting flag F_(K1) (i). Inthis setting routine, the value of the inhibiting flag F_(K1) (i) is setat 1 only when all the results of determination in Steps S627 to S631are Yes.

In the setting section 95, an inhibiting flag F_(K2) (i) is set at 1when the wheel slip is excessive. The setting section 95 includes an ANDcircuit 98, the output of which is supplied to the switch 92 as theinhibiting flag F_(K2) (i). If all of the two input conditions of theAND circuit 98 are met, that is, if all the inputs are on, the value ofthe inhibiting flag F_(K2) (i) is set at 1. If either of the inputconditions is off, the value of the inhibiting flag F_(K2) (i) is resetat 0. One input condition is on when the wheel slip factor S_(L) (i) ishigher than an allowable slip factor S_(LMAX) (i), and the other inputcondition is on when the control mode M(i) is the intensify-pressuremode.

When the switch 92 receives the flag F_(K2) (i), it is shifted from theposition shown in FIG. 29, whereupon the pulse width W_(PLS1) (i) is setat 0.

Referring to FIG. 31, there is shown in detail a setting routine for theinhibiting flag F_(K2) (i). In this setting routine, it is firstdetermined whether or not the value of the aforementioned on-off flagF_(ymc) is 1, that is, whether or not the vehicle is under the yawmoment control (Step S634). If the result of determination in Step S634is Yes, it is determined whether or not the wheel (intensify-pressurewheel) for which the control mode M(i) is the intensify-pressure mode issubjected to the brake pressure control using ABS (Step S635). A flagF_(ABS) (i), mentioned later, is used for the determination in StepS635. Therefore, the setting section 95 is also supplied with the flagF_(ABS) (i) as shown in FIG. 29.

If the result of determination in Step S635 is Yes, the slip factor ofthe wheel in the intensify-pressure mode at the start of the brakepressure control using ABS is held as a determination slip factorS_(LST) (i) (Step S636). If the result of determination in Step S635 isNo, Step S636 is not executed. The brake pressure control using ABS willbe described later.

If the result of determination in Step S634 is No, that is, if thevehicle is not under the yaw moment control, the determination slipfactor S_(LST) is reset at 0 (Step S637).

After any of Steps S635, S636, and S637, the next Step S638 is executed.In Step S638, it is determined whether or not the determination slipfactor S_(LST) (i) is 0. If the result of this determination is Yes,that is, if the intensify-pressure wheel is not under the brake pressurecontrol using ABS, the allowable slip factor S_(LMAX) (i) is computed(Step S639). Specifically, the allowable slip factor S_(LMAX) (i) isread from a map shown in FIG. 32 in accordance with the required yawmoment γ_(d). As seen from FIG. 32, the allowable slip factor S_(LMAX)(i) has a characteristic such that it increases at a predetermined rateas the required yaw moment γ_(d) increases, and its maximum value is setat 20%.

Next, it is determined whether or not the slip factor S_(L) (i) of thewheel in the intensify-pressure mode is higher than the allowable slipfactor S_(LMAX) (i) (Step S641). If the result of this determination isYes, the inhibiting flag F_(K2) (i) is set at 1 (Step S642). If theresult of determination in Step S641 is No, the inhibiting flag F_(K2)(i) is set at 0 (Step S643).

If the result of determination in Step S638 is Yes, that is, if thewheel in the intensify-pressure mode is under the brake pressure controlusing ABS, the map from which the allowable slip factor S_(LMAX) (i) isread is modified (Step S640). Specifically, the map of FIG. 32 isreplaced by a map shown in FIG. 33 in Step S640. On the map of FIG. 33,the maximum value of the allowable slip factor S_(LMAX) (i) is adjustedto the determination slip factor S_(LST) (i) (or 95% of S_(LST) (i)).Also, its increase rate of the allowable slip factor S_(LMAX) (i) ischanged in accordance with the determination slip factor S_(LST) (i).

When the wheel in the intensify-pressure mode is under the brakepressure control using ABS as described above, the determination slipfactor S_(LST) (i) is adjusted to the allowable slip factor S_(LMAX)(i). Thereupon, the result of determination in Step S641 is Yes, so thatthe inhibiting flag F_(K2) (i) is set at 1 (Step S642).

In the setting section 96 (see FIG. 29), an inhibiting flag F_(K3) isset at 1 to prevent overshooting of yaw moment control when the absolutevalue of the required yaw moment γ_(d) decreases at a rate higher than apredetermined rate, that is, when the condition is met. On the otherhand, if the above condition is not met, the inhibiting flag F_(K3) isreset at 0. The inhibiting flag F_(K3) is supplied from the settingsection 96 to the switch 93, and the switch 93 is shifted in accordancewith the value of the inhibiting flag F_(K3). When the inhibiting flagF_(K3) is set at 1, the switch 93 is shifted from the position shown inFIG. 29, whereupon the value of the pulse width W_(PLS1) (i) is reset at0.

Referring to FIG. 34, there is shown in detail a setting routine for theinhibiting flag F_(K3). In this setting routine, the required yaw momentγ_(d) is read first (Step S644), and a derivative Δγ_(d) of the absolutevalue of the required yaw moment γ_(d) is computed (Step S645). Further,the derivative Δγ_(d) is filtered (f_(c) =2 Hz) (Step S646).

The processing in Steps S645 and S646 can be expressed as follows:

    Δγ.sub.d =LPF(|γ.sub.d |-|γ.sub.dm |)           (41)

where γ_(dm) is the required yaw moment computed in the precedingroutine.

Then, it is determined whether or not Δγ_(d) is larger than an overshootdetermination value Ay, (e.g., -125 kgm/s²) (Step S647). If the resultof determination in Step S647 is Yes, 1 is set in the inhibiting flagF_(K3) (Step S648). If the result of determination in Step S647 is No, 0is set in the inhibiting flag F_(K3) (Step S649).

Referring again to FIG. 23, the yaw moment control block includes apre-pressurization control determination section 100. In thisdetermination section 100, the respective values of pre-pressurizationflags F_(PRE1) and F_(PRE2) for controlling the operation of theaforesaid pumps 16 and 17, the respective solenoid valves (inlet andoutlet valves 12 and 13), and the cutoff valves 19 and 20 are set inadvance of the start of the yaw moment control. Specifically, if theabsolute value of the required yaw moment or the maximum yaw ratedeviation Δγ_(MAX) is greater than a predetermined value so that the yawmoment control is permitted to start, the pre-pressurization flagF_(PRE1) or F_(PRE2) is set at 1, and this state is kept for a fixedperiod of duration (e.g., 96 msec). When the yaw moment control isstarted during this period, the pre-pressurization flag F_(PRE1) orF_(PRE2) is set at 0 at the start time. The pre-pressurization flagF_(PRE1) is provided for a clockwise turn of the vehicle, and F_(PRE2)is provided for a counterclockwise turn.

As shown in FIG. 23, the yaw moment control block includes aforced-modification section 111 for valve control signal. The details ofthe forced-modification section 111 are shown in FIG. 35. In theforced-modification section 111, the pulse width W_(PLS) (i) and theactuation mode M_(PLS) (i) are compulsorily modified depending onvarious conditions. These pulse width W_(PLS) (i) and the actuation modeM_(PLS) (i) are outputted from the forced-modification section 111 as apulse width W_(y) (i) and an actuation mode M_(y) (i).

Specifically, as seen from FIG. 35, the actuation mode M_(PLS) (i) isoutputted as the actuation mode M_(y) (i) after passing through switches112 to 117. These switches 112 to 117 are supplied with flags andshifted in accordance with the respective values of the flags.

The switch 112 is shifted in accordance with the value of a hold flagF_(HLD) (i) delivered from a hold determination section 118. In thedetermination section 118, the hold flag F_(HLD) (i) for the wheel forwhich the noncontrol mode is established is set at 1 when the pump 16 or17 is actuated (or if an actuation flag F_(MTR), mentioned later, is setat 1) with the vehicle not braked (F_(b) =0). In this case, the switch112 is shifted from the position shown in FIG. 35, and only theactuation mode M_(PLS) (i) having the noncontrol mode is compulsorilychanged to the hold mode. If the values of all the hold flags F_(HLD)(i) are reset at 0, the actuation mode M_(PLS) (i) is outputted directlyfrom the switch 112. Therefore, even if the pump 16 or 17 is actuatedwith the vehicle not braked, the noncontrol mode of the actuation modeM_(PLS) (i) is compulsorily changed to the hold mode, so that thepressure of discharge from the pump 16 or 17 cannot be supplied to thewheel brakes for the wheels.

The switch 113 is shifted in accordance with the value of a terminationflag F_(FIN) (i) delivered from a termination control determinationsection 119. When the yaw moment control is terminated and the on-offflag F_(ymc) is reset at 0, the determination section 119 sets thetermination flag F_(FIN) (i) at 1 periodically for a fixed time period(e.g., 340 msec). Specifically, the termination flag F_(FIN) (i) is setat 1 for a predetermined time (e.g., 16 msec) with every predeterminedcycle (e.g., 40 msec). The termination flag F_(FIN) (i) is also used forthe control of opening/closing of the cutoff valves 19 and 20 asdescribed later.

If the termination flag F_(FIN) (i) is set at 1, the switch 113 isshifted from the position shown in FIG. 35. Therefore, among theactuation modes M_(PLS) (i), the actuation mode of the target wheelsubjected to yaw moment control is compulsorily changed to the holdmode. When the values of all the termination flags F_(FIN) (i) are resetat 0, the actuation mode M_(PLS) (i) is outputted directly from theswitch 113. After the termination of yaw moment control, when theactuation mode of the target wheel to be controlled is changed to thehold mode periodically, the brake pressure of the target wheel to becontrolled does not change suddenly, so that the behavior of the vehicleis stabilized.

The switch 114 is shifted in accordance with the values of thepre-pressurization flags F_(PRE1) and F_(PRE2) delivered from thepre-pressurization control determination section 100. When thepre-pressurization flag F_(PRE1) or F_(PRE2) is set at 1, the switch 114is shifted from the position shown in FIG. 35. In this case, among theactuation modes M_(PLS) (i), the actuation mode of the target wheel tobe controlled is compulsorily changed to the hold mode. When both of thepre-pressurization flags F_(PRE1) and F_(PRE2) are reset at 0, theactuation mode M_(PLS) (i) is outputted directly from the switch 114.

FIG. 23 shows that the control mode M(i) and the actuation mode M_(PLS)(1) are set by receiving the supply of the on-off flag F_(ymc) in thedetermination section 88. However, the control mode M(i) and theactuation mode M_(PLS) (i) are set without regard to the value of theon-off flag F_(ymc), as seen from FIGS. 26 and 28. Even if thepre-pressurization control (mentioned later) is started, therefore, thebrake pressure for the target wheel to be controlled cannot be adverselyaffected before the start of the yaw moment control.

The switch 115 is shifted in accordance with the value of a release flagF_(RP) set in a determination section 120 in which the release of brakepedal is determined. When the brake pedal 3 is released during the timewhen the yaw moment control is carried out with the vehicle braked, thedetermination section 120 sets the release flag F_(RP) at 1 for apredetermined time (e.g., 64 msec). In this case, the switch 115 isshifted from the position shown in FIG. 35, and among the actuationmodes M_(PLS) (i), the actuation mode of the target wheel to becontrolled is compulsorily changed to a reduce-pressure mode. When therelease flag F_(RP) is reset at 0, the switch 115 outputs the actuationmode M_(PLS) (i) as it is.

As can be seen from FIG. 35, the release flag F_(RP) is also supplied tothe switch 121. When the release flag F_(RP) is set at 1, the switch 121is shifted from the position shown in FIG. 35, and the value of thepulse width W_(PLS) (i), that is, the pulse width W_(y) (i) iscompulsorily modified into the control period T (=8 msec). When therelease flag F_(RP) is reset at 0, the pulse width W_(PLS) (i) isoutputted directly from the switch 121 as the pulse width W_(y) (i).

The switch 116 is shifted in accordance with the value of the augmenteddepression flag F_(PP) delivered from a determination section 122 fordetermining the augmented depression of brake pedal. The augmenteddepression flag F_(PP) is set in accordance with the aforementionedroutine shown in FIG. 6. When the augmented depression flag F_(PP) isset at 1, the switch 116 is shifted from the position shown in FIG. 35,and all the actuation modes M_(PLS) (i) are compulsorily modified intothe noncontrol mode. When the augmented depression flag F_(PP) is resetat 0, the actuation mode M_(PLS) (i) is outputted directly from theswitch 116. When the actuation modes of all the wheels are compulsorilymodified into the noncontrol mode, the driver's brake pedal operation isreflected in the brake pressures of all the wheels.

The switch 117 is shifted in accordance with the value of a reverse flagF_(rev) delivered from a reverse determination section 123. The reversedetermination section 123 sets the reverse flag F_(rev) at 1 when thereverse gear is selected in the transmission of the vehicle, and resetsthe reverse flag F_(rev) at 0 when the forward gear is selected. Whenthe reverse flag F_(rev) is set at 1, the switch 117 is shifted from theposition shown in FIG. 35, and all the actuation modes M_(PLS) (i) arecompulsorily modified into the noncontrol mode. When the reverse flagF_(rev) is reset at 0, the actuation mode M_(PLS) (i) is outputteddirectly from the switch 117 as the actuation mode M_(y) (i).

As can be seen from FIG. 23, the output from the valve control signalforced-modification section 111, that is, the actuation mode M_(y) (i),and the output from the pre-pressurization control determination section100, that is, the pre-pressurization flags F_(PRE1) and F_(PRE2) arealso supplied to an actuation determination section 124. FIGS. 36 to 39show the details of the determination section 124.

First, the actuation determination section 124 includes a determinationcircuit 125 shown in FIG. 36. In this determination circuit 125,respective request flags for requesting the actuation of the cutoffvalves 19 and 20 and the motor 18 are set for the wheel cylinder of eachwheel. The determination circuit 125 has two AND circuits 126 and 127.When the brake flag F_(b) is set at 1 and the actuation mode M_(y) (i)is the intensify-pressure mode, all the inputs of one AND circuit areon. In this case, the wheel number i in the intensify-pressure mode isoutputted from the AND circuit 126 to an OR circuit 128.

When the brake flag F_(b) is reset at 0 and the actuation mode M_(y) (i)is not the noncontrol mode, all the inputs of the other AND circuit 127are on. In this case, the wheel number i which is not in the noncontrolmode is outputted from the AND circuit 127 to the OR circuit 128. Thatis, as seen from FIG. 36, the one input condition of the AND circuit 127is inverted by a NOT circuit 129.

When the OR circuit 128 receives outputs from the AND circuits 126 and127, it outputs a request flag F_(MON) (i) for requesting the actuationof the motor 18. In this case, the request flag F_(MON) (i)corresponding to the wheel number i supplied to the OR circuit 128 isset at 1.

The output from the OR circuit 128 is also supplied to a set terminal ofa flip-flop 130. To a reset terminal of the flip-flop 130, among theactuation modes M_(y) (i), a reset signal corresponding to the wheelnumber i in the noncontrol mode is supplied.

When the request flag F_(MON) (i) is supplied to the set terminal of theflip-flop 130, the flip-flop 130 outputs a request flag F_(COV) (i) forrequesting the actuation of the cutoff valves 19 and 20. In this case,among the request flags F_(COV) (i), the request flag F_(COV) (i) of thewheel number i corresponding to the request flag F_(MON) (i) whose valueis set at 1 is set at 1. When the flip-flop 130 receives the resetsignal, all the request flags F_(COV) (i) are reset at 0.

Next, the actuation determination section 124 includes a determinationcircuit 131 shown in FIG. 37. The determination circuit 131 has an ORcircuit 132. The OR circuit 132 outputs 1 as the value of the actuationflag F_(VD1) for actuating the cutoff valve 19, if any of the values ofrequest flags F_(COV) (1) and F_(COV) (4), termination flags F_(FIN) (1)and F_(FIN) (4), and pre-pressurization flag F_(PRE1), which areassociated with the cutoff valve 19 on the side of the front-left andrear-right wheels FW_(L) and RW_(R), is set at 1.

Switches 133 and 134 are disposed on the output line from the OR circuit132. The switches 133 and 134 are shifted by the augmented depressionflag F_(PP) and the reverse flag F_(REV), respectively. That is, if theaugmented depression flag F_(PP) or the reverse flag F_(REV) is set at1, the switch 133 or the switch 134 is shifted from the position shownin FIG. 37. In this case, even if the actuation flag F_(VD1) is set at1, the OR circuit 132 resets the actuation flag FVD₁ at 0 (noncontrolmode).

Further, the actuation determination section 124 includes adetermination circuit 135 shown in FIG. 38. This determination circuit135 has the same construction and function as those of the determinationcircuit 131 of FIG. 37, but differs from the determination circuit 131in the following respect. An OR circuit 136 in the determination circuit135 outputs 1 as the value of the actuation flag F_(VD2) for actuatingthe cutoff valve 20, if any of the values of request flags F_(COV) (2)and F_(COV) (3), termination flags F_(FIN) (2) and F_(FIN) (3), andpre-pressurization flag F_(PRE2), which are associated with the cutoffvalve 20 on the side of the front-right and rear-left wheels FW_(R) andRW_(L), is set at 1.

The actuation determination section 124 further includes a determinationcircuit shown in FIG. 39. This determination section has an OR circuit139, which outputs 1 as the value of the actuation flag F_(MTR) foractuating the motor, if any of the values of request flags F_(MON) (i)is set at 1 or if at least either of the pre-pressurization flagsF_(PRE1) and F_(PRE2) is set at 1 and this state is continued.

Cooperation Control for ABS

When the actuation mode M_(y) (i), pulse width W_(y) (i), actuationflags F_(VD1) and F_(VD2), and flag F_(MTR) are set in theabove-described yaw moment control block 78 (see FIG. 3), thecooperation control for ABS is carried out. This cooperation control isshown in the determination block 78a in FIG. 3 and Step S7 in FIG. 4.

In the cooperation control, when the brake pressure control using ABS isstarted, the actuation modes M_(ABS) (i) and pulse widths W_(ABS) (i)are set to execute yaw moment control in cooperation with the brakepressure control by ABS. A detailed description of the setting of theactuation modes M_(ABS) (i) and the pulse widths W_(ABS) (i) will beomitted. It is to be noted, however, that the control by the aforesaidinhibitory section 90 (FIG. 29) and forced-modification 111 (FIG. 35) isalso applied to the actuation modes M_(ABS) (i) and pulse widths W_(ABS)(i).

The following is a description of a function of the cooperation control.In the case where the vehicle requires the turning moment or restorationmoment while it is turning under the brake pressure control using ABS,the actuation modes M_(ABS) (i) and pulse widths W_(ABS) (i) are set inaccordance with the cooperation routine shown in FIG. 40 in thecooperation control.

First, in Step S701, it is determined whether or not the vehicle isunder the brake pressure control using ABS. A flag F_(ABS) (i) is usedfor the determination, and is set at 1 when the corresponding wheelbecomes the target wheel to be subjected to brake pressure control usingABS. That is, the flag F_(ABS) (i) is set at 1 in accordance with thetrend of the slip factor of the wheel in an ABS control routine (notshown).

If the result of determination in Step S701 is Yes, it is determinedwhether or not the control execution flag F_(CUS) or F_(COS) is 1 (StepS702). If the result of determination in Step S702 is Yes, that is, ifit is concluded that the vehicle requires the turning moment orrestoration moment while it is turning, the actuation modes M_(ABS) (i)and pulse widths W_(ABS) (i) are set in the following manner in StepS703, the next stage.

In the case where the yaw moment control is executed for a diagonal pairof wheels:

(1) To give the turning moment to the vehicle, the actuation modeM_(ABS) (i) of the inside front wheel FW, viewed in the vehicle turndirection, is set in the reduce-pressure mode, and the pulse widthW_(ABS) (i) is set at the same value as the pulse width of the outsidefront wheel FW.

(2) To give the restoration moment to the vehicle, the actuation modeM_(ABS) (i) of the outside rear wheel RW is set in the reduce-pressuremode, and the pulse width W_(ABS) (i) is set at the same value as thepulse width of the inside rear wheel.

The yaw moment control can be executed to a parallel pair of wheels onthe front or rear side, as well as to the diagonal pair. In executingthe yaw moment control on the basis of the difference in braking forcebetween left- and right-hand wheels, the restoration moment is given tothe vehicle by setting the actuation mode M_(ABS) (i) of the outsidewheel and the actuation mode M_(ABS) (i) of the inside wheel in theintensify-pressure mode and the reduce-pressure mode, respectively. Onthe other hand, the turning moment is given to the vehicle by settingthe actuation mode M_(ABS) (i) of the outside wheel and the actuationmode M_(ABS) (i) of the inside wheel in the reduce-pressure mode and theintensify-pressure mode, respectively.

When the yaw moment control is executed for the left- and right-handrear wheels, a front wheel can be added as the target wheel to becontrolled. That is, to further give the turning moment to the vehicle,the actuation mode M_(ABS) (i) of the outside front wheel is set in thereduce-pressure mode, and the pulse width W_(ABS) (i) is set at the samevalue as the pulse width of the outside rear wheel.

Even when the yaw moment control is executed for the left- andright-hand side wheels, a rear wheel can be added as the target wheel tobe controlled. In this case, to further give the restoration moment tothe vehicle, the actuation mode M_(ABS) (i). of the inside rear wheel isset in the reduce-pressure mode, and the pulse width W_(ABS) (i) is setat the same value as the pulse width of the inside front wheel.

Selection of Valve Control Signal

After the aforesaid cooperation routine (Step S7 in FIG. 4) has beenexecuted, in the next Step S8, that is, in the selecting circuit 140shown in FIG. 41, a valve control signal selection routine is executed.FIG. 41 also shows sections 141 and 142 for carrying out the routine ofFIG. 40.

The selecting circuit 140 is provided with four switches 143, 144, 145and 146. The switch 143 is supplied with the actuation mode M_(ABS) (i)outputted from the section 141 and the actuation mode M_(y) (i) setduring the aforesaid yaw moment control. The switch 144 is supplied withthe pulse width W_(ABS) (i) outputted from the section 142 and the pulsewidth W_(y) (i) set during the yaw moment control. The switch 145 issupplied with the actuation flags F_(VD1) and F_(VD2) set during the yawmoment control, and a value of 0 for resetting these flags F_(VD1) andF_(VD2). The switch 146 is supplied with the actuation flag F_(MTR), setduring the yaw moment control, through an OR circuit 147, and also withan actuation flag F_(MABS). The actuation flag F_(MABS) is also suppliedto the OR circuit 147. The actuation flag F_(MABS) is set at 1 when thebrake pressure control using ABS is started.

The switches 143 to 146 are shifted in accordance with he values offlags delivered from a determination section 148. The determinationsection 148 includes an OR circuit 149. When the brake pressure controlusing ABS is executed for three or more wheels, or when the actuationmode M_(y) (i) for the yaw moment control is not the reduce-pressuremode, among flags F_(MY) (i) delivered from the OR circuit 149, the flagF_(MY) (i) corresponding to the wheel in the reduce-pressure mode is setat 1. The flag F_(MY) (i) is supplied to an AND circuit 150. When thebrake pressure control using ABS is executed for three or more wheels,the switches 145 and 146 are supplied with a flag F_(ABS3) whose valueis set at 1.

The AND circuit, 150 is supplied with a flag F_(MZ) (i) as well as theflag F_(MY) (i). The flag F_(MZ) (i) corresponding to the wheel number inot in the noncontrol mode, among the actuation modes M_(ABS) (i) forthe cooperation control, is set at 1. A flag F_(M).sbsb.--_(A) (i) isdelivered from the AND circuit 150, and supplied to the switches 143 and144. Among the flags F_(M).sbsb.--_(A) (i), the flag F_(M).sbsb.--_(A)(i) corresponding to the wheel number i for which both the flags F_(MY)(i) and F_(MZ) (i) are set at 1 is set at 1. That is, the flagF_(M).sbsb.--_(A) (i) corresponding to the wheel number 1 in thereduce-pressure mode is set at 1.

When the brake pressure control using ABS is being actuated for three ormore wheels of the vehicle, the flag F_(ABS3) supplied from thedetermination section 148 to the switches 145 and 146 is set at 1.Therefore, the switches 145 and 146 are shifted from the positions shownin FIG. 41. In this case, actuation flags F_(V1) and F_(V2) deliveredfrom the switch 145 are both set at 1, and the switch 146 outputs theactuation flag F_(MABS) as an actuation flag F_(M). On the other hand,when the flag F_(ABS3) is reset at 0, the switch 145 outputs theactuation flags F_(VD1) and F_(VD2) as F_(V1) and F_(V2), respectively,and the switch 146 outputs the actuation flag F_(MTR) as F_(M). Sincethe actuation flag F_(MABS) is supplied to the switch 146 through the ORcircuit 147, the actuation flag F_(M) delivered from the switch 146 isset at 1 when either of the actuation flags F_(MABS) and F_(MTR) is setat 1 without respect to the shifting of the switch 146.

If input conditions of the AND circuit 150 are met, the switch 143outputs one of the actuation modes M_(ABS) (i) and M_(y) (i) as theactuation mode MM(i) in accordance with the value of flagF_(M).sbsb.--_(A) (i) delivered from the AND circuit 150 to the switches143 and 144 and according to the wheel number i. Also, the switch 144outputs one of the pulse widths W_(ABS) (i) and W_(y) (i) as the pulsewidth WW(i).

Initial setting for Drive Signal

When the actuation mode MM(i) and the pulse width WW(i) are deliveredfrom the valve control signal selecting circuit 140, they are suppliedto the drive signal initial setting section 151 in FIG. 3 (Step S9 inFIG. 4). In this section 151, the actuation mode MM(i) and the pulsewidth WW(i) are set as an actual actuation mode M_(EXE) (i) and anactual pulse width W_(EXE) (i), respectively, and initial values aregiven to the actual actuation mode M_(EXE) (i) and the actual pulsewidth W_(EXE) (i), individually.

Step S9 is shown in detail in FIG. 42. As seen from FIG. 42, after aninterruption inhibiting process is first executed (Step S901), theactuation mode MM(i) is discriminated (Step S902).

If the result of discrimination in Step S902 indicates the noncontrolmode, the intensify-pressure mode is established as the actual actuationmode M_(EXE) (i), and the control period T (=8 msec) for the mainroutine is set as the actual pulse width W_(EXE) (i) (Step S903). Afteran interruption permitting process is executed (Step S904), the routineconcerned is finished.

If the result of discrimination in Step S902 indicates theintensify-pressure mode, it is determined whether or not he actualactuation mode M_(EXE) (i) is the intensify-pressure mode (Step S905).Since the actual actuation mode M_(EXE) (i) is not established at thispoint of time, the result of this determination is No. In this case, theactuation mode MM(i) or intensify-pressure mode is established as theactual actuation mode M_(EXE) (i), and the pulse width WW(i) is set asthe actual pulse width W_(EXE) (i) (Step S906). Thereafter, the routineconcerned is terminated after the execution of Step S904.

If it is concluded in Step S902 that the intensify-pressure mode ismaintained when routine in the next cycle is executed repeatedly, theresult of determination in Step S905 is Yes. In this case, it isdetermined whether or not the pulse width WW(i) is larger than theactual pulse width W_(EXE) (i) (Step S907). Since the main routine isexecuted with every control period T, the pulse width WW(i) is newly setwith every control period T. As mentioned later, however, the actualpulse width W_(EXE) (i) decreases as the inlet or outlet valve 12 or 13is actually actuated. If it is concluded in Step S907 that the newly setpulse width WW(i) is longer than the remaining actual pulse widthW_(EXE) (i) at the present point of time, therefore, a new pulse widthWW(i) is set as the actual pulse width W_(EXE) (i) (Step S908). If theresult of determination in Step S907 is No, however, the remainingactual pulse width W_(EXE) (i) is maintained without resetting the newpulse width WW(i) as the actual pulse width W_(EXE) (i).

If the result of discrimination in Step S902 indicates thereduce-pressure mode, on the other hand, Steps S909 to S912 areexecuted, whereupon the actual actuation mode M_(EXE) (i) and the actualpulse width W_(EXE) (i) are set in the same manner as in the case of theintensify-pressure mode.

If the result of discrimination in Step S902 indicates the hold mode,moreover, the hold mode is established as the actual actuation modeM_(EXE) (i) (Step S913).

Drive Signal Outputting

When the actual actuation mode M_(EXE) (i) and the actual pulse widthW_(EXE) (i) are set in the manner described above, they are deliveredfrom the drive signal initial setting section 151 to the valve actuatingsection 152, as shown in FIG. 3, and Step S10 (FIG. 4) is executed.

In Step S10, drive signals for driving the cutoff valves 19 and 20 andthe motor 18 are also outputted in accordance with the actuation flagsF_(V1) and F_(V2) and the flag F_(M) set in the foregoing control signalselection routine, as well as the actual actuation mode M_(EXE) (i) andthe actual pulse width W_(EXE) (i).

A drive signal for closing the cutoff valve 19 is outputted if theactuation flag F_(V1) is set at 1, while a drive signal for closing thecutoff valve 20 is outputted if the actuation flag F_(V2) is set at 1.If the actuation flags F_(V1) and F_(V2) are reset at 0, in contrastwith this, the cutoff valves 19 and 20 are kept open. In the case wherethe motor actuation flag F_(M) is set at 1, on the other hand, a drivesignal for actuating the motor 18 is outputted. In the case where theactuation flag F_(M) is reset at 0, the motor 18 is not actuated.

Actuation of Inlet and Outlet Valves

When the valve actuating section 152 is supplied with the actualactuation mode M_(EXE) (i) and the actual pulse width W_(EXE) (i), itactuates the inlet and outlet valves 12 and 13 according to an actuationroutine shown in FIG. 43. The actuation routine of FIG. 43 is executedindependently of the main routine of FIG. 4, and its execution period is1 msec.

In the actuation routine, the actual actuation mode M_(EXE) (i) is firstdiscriminated (Step S1001). If the actual actuation mode M_(EXE) (i) isthe intensify-pressure mode, in this discrimination, it is determinedwhether or not the actual pulse width W_(EXE) (i) is greater than 0(Step S1002). If the result of determination in Step S1002 is Yes, theinlet and outlet valves 12 and 13 for the corresponding wheel are openedand closed, respectively, and the actual pulse width W_(EXE) (i) isreduced by a margin for its execution period (Step S1003). When StepS1003 is carried out, therefore, the brake pressure for the wheelconcerned is increased if the motor 18 is already actuated and if thecorresponding cutoff valve 19 or 20 is closed.

If the result of determination in Step S1002 becomes No in the conditionthat the actuation routine is executed repeatedly with theintensify-pressure mode maintained as the actual actuation mode M_(EXE)(i), both the inlet and outlet valves 12 and 13 for the wheel concernedare closed, and the pressure-hold mode is established as the actualactuation mode M_(EXE) (i) (Step S1004).

If it is concluded in Step S1001 that the actual actuation mode M_(EXE)(i) is the reduce-pressure mode, it is determined whether or not theactual pulse width W_(EXE) (i) is greater than 0 (Step S1005). If theresult of determination in Step S1005 is Yes, the inlet and outletvalves 12 and 13 for the wheel concerned are closed and opened,respectively, and the actual pulse width W_(EXE) (i) is reduced by amargin for its execution period (Step S1006). When Step S1006 is carriedout, therefore, the brake pressure for the wheel concerned is decreased.

If the result of determination in Step S1005 becomes No in the conditionthat the actuation routine is executed repeatedly with thereduce-pressure mode maintained as the actual actuation mode M_(EXE)(i), both the inlet and outlet valves 12 and 13 for the wheel concernedare closed, and the pressure-hold mode is established as the actualactuation mode M_(EXE) (i) (Step S1007).

If it is concluded in Step S1001 that the actual actuation mode M_(EXE)(i) is the pressure-hold mode, both the inlet and outlet valves 12 and13 for the wheel concerned are closed (Step S1008).

Referring to FIG. 44, the relations between the actuation mode MM(i),pulse width WW(i), actual actuation mode M_(EXE) (i), and actual pulsewidth W_(EXE) (i) are shown using a time chart.

Effectiveness of Yaw Moment Control

When yaw moment control is applied to diagonal wheels of the vehicle:

Let it be supposed that the vehicle is running and the main routine ofFIG. 4 is being executed repeatedly. It can be concluded that thevehicle is turning clockwise if the turn flag F_(d), which indicates aturn of the vehicle in accordance with the steering wheel angle θ andthe yaw rate γ, is set at 1 in Step S3 or in a turn determinationroutine shown in FIG. 8.

(a) Clockwise turn of vehicle

Thereafter, the required yaw moment γ_(d) is obtained by executing StepS5 in the main routine. When the yaw moment control is executed in StepS6, the control mode selection routine is executed on condition that theon-off flag F_(ymc) (see the determination circuit in FIG. 24) is setat 1. That is, the control mode M(i) for each wheel is set in accordancewith the selection routine shown in FIG. 26.

Since the vehicle is supposed to be turning clockwise, the result ofdetermination in Step S601, in the selection routine of FIG. 26, is Yes,whereupon Step S602 and the subsequent steps are carried out.

(b) Clockwise turn of understeer-prone vehicle

If the result of determination in Step S602 is Yes, that is, if thevehicle has a marked tendency to understeer with the control executionflag F_(CUS) being set at 1, the reduce-pressure and intensify-pressuremodes are established as the control modes M(1) and M(4) for thefront-left (outside front) wheel FW_(L) and the rear-right (inside rear)wheel RW_(R), respectively, and the noncontrol mode as the control modesM(2) and M(3) for the other two wheels (see Table 1 and Step S603).

Based on the control mode M(i) and the required yaw moment γ_(d) foreach wheel, thereafter, the actuation mode M_(PLS) (1) is set (see thesetting routine in FIG. 28), and the pulse width W_(PLS) (i) for eachwheel is set. The actuation mode M_(PLS) (i) and the pulse width W_(PLS)(i) are changed into the actuation mode M_(y) (i) and the pulse widthW_(y) (i), respectively, by the inhibitory section 90 and theforced-modification section 111 in FIG. 23.

In the determination circuit 125 (FIG. 36) of the actuationdetermination section 124 of FIG. 23 (the determination circuits shownin FIGS. 36 to 39), on the other hand, of the request flag F_(MON) (i)outputted through the AND circuit 126 and the OR circuit 128 and of therequest flag F_(COV) (i) outputted through the flip-flop 130, therequest flag corresponding to the target wheel to be controlled is setat 1 in the case where the brake flag F_(b) is set at 1 (brakes on) andthe actuation mode M_(y) (i) is the intensify-pressure mode.

Specifically, when the vehicle is turning clockwise showing a markedtendency to understeer with the brake pedal 3 depressed, of the outputsfrom the determination circuit 125 (FIG. 36), F_(MON) (4) and F_(COV)(4) are set at 1. Also, the actuation flag F_(VD1) outputted from thedetermination circuit 131 (OR circuit 132) in FIG. 37 is set at 1.Further, the actuation flag F_(MTR) outputted from the determinationcircuit in FIG. 39, that is, the OR circuit 139 is set at 1. Since therequest flags F_(COV) (2) and F_(COV) (3) are both reset at 0, theactuation flag F_(VD2) outputted from the determination circuit 135 (ORcircuit 136) in FIG. 38 is reset at 0.

Thereafter, therefore, the actuation flags F_(V1) and F_(M) outputtedfrom the control signal selecting section 140 in FIG. 3 (switches 145and 146 in FIG. 41) are set at 1, and the actuation flag F_(V2) is resetat 0. These flags are supplied as drive signals to the cutoff valves 19and 20 and the motor 18. Thus, the motor 18 is actuated in a state suchthat the cutoff valve 19, which is associated with the wheel brakes forthe front-left and rear-right wheels FW_(L) and RW_(R), is closed, andthe cutoff valve 20, which is associated with the wheel brakes for thefront-right and rear-left wheels FW_(R) and RW_(L), is left open. As themotor 18 is driven in this manner, a pressurized fluid is dischargedfrom the pumps 16 and 17.

When the brake pedal 3 is not depressed, that is, when the vehicle isnot braked, the control modes M(1) and M(4) for the front-left andrear-right wheels FW_(L) and RW_(R), respectively, are not thenoncontrol mode, so that the request flags F_(MON) (1) and F_(MON) (4)delivered from the OR circuit 128 of the determination circuit 125 areset at 1, and the request flags F_(COV) (1) and F_(COV) (4) deliveredfrom the flip-flop 130 are set at 1. Also in this case, therefore, theactuation flag F_(MTR) is set at 1, so that the motor 18 or the pumps 16and 17 are actuated. Only the actuation flag F_(VD1) is set at 1,whereupon only the cutoff valve 19 is closed.

If the actuation mode M_(PLS) (i) is processed in theforced-modification section 111 (FIG. 23) when the vehicle is notbraked, however, the flag F_(HLD) (i) delivered from the holddetermination section 118 (FIG. 35) is set at 1. In this case, theswitch 112 is shifted, and the actuation mode M_(PLS) (i) iscompulsorily changed from the noncontrol mode to the hold mode.

When the vehicle is not braked (F_(b) =0), the correction value C_(pi)of the required yaw moment γ_(d) to be computed (see FIG. 10) is set at1.5, which is greater than 1.0 for the case where the vehicle is braked,so that the required yaw moment γ_(d) is increased. This increaseshortens the pulse period T_(PLS) during which the actuation modeM_(PLS) (i) or M_(y) (i) is executed. As a result, the pressure increaseor decrease is executed positively if the actuation mode M_(y) (i) isthe intensify-pressure or reduce-pressure mode.

Thereafter, the actuation mode M_(y) (i) and the pulse width W_(y) (i)are set as the actuation mode MM(i) and the pulse width WW(i),respectively, by the control signal selecting section 140, as mentionedbefore, and moreover, the actual actuation mode M_(EXE) (i) and theactual pulse width W_(EXE) (i) are set in accordance with the setvalues. As a result, the corresponding inlet and outlet valves 12 and 13of the vehicle are actuated in accordance with the actual actuation modeM_(EXE) (i) and the actual pulse width W_(EXE) (i) (see the actuationroutine of FIG. 43).

Specifically, the actual actuation mode M_(EXE) (1) for the front-leftwheel FW_(L) is the reduce-pressure mode when the vehicle is braked asit makes a clockwise turn showing a marked tendency to understeer.Accordingly, the inlet and outlet valves 12 and 13 for the front-leftwheel FW_(L) are closed and opened, respectively (Step S1006 in FIG.43), so that the brake pressure for the front-left wheel FW_(L) isdecreased. On the other hand, the actual actuation mode M_(EXE) (4) forthe rear-right wheel RW_(R) is the intensify-pressure mode, so that theinlet and outlet valves 12 and 13 for the rear-right wheel RW_(R) areopened and closed, respectively (Step S1003 in FIG. 43). At this pointof time, the cutoff valve 19 is closed, and the pumps 16 and 17 areactuated by the motor 18, as mentioned before. Accordingly, the pressurein the branch brake line 8 (see FIG. 1), which leads to the wheel brakefor the rear-right wheel RW_(R), is already raised independently of themaster cylinder pressure, so that the wheel brake for the rear-rightwheel RW_(R) is supplied with the pressurized fluid from the brakebranch line 8 through the inlet valve 12. Thus, the brake pressure forthe rear-right wheel RW_(R) is increased.

FIG. 45 shows braking force/cornering force characteristics of thevehicle compared with the slip factor. If the brake pressure or brakingforce F_(x) for a wheel decreases, as seen from FIG. 45, the slip factoralso decreases within a slip factor range for the case where the vehicleis in normal running conditions. If the cornering force F_(y) increases,in contrast with this, the slip factor also increases. On the otherhand, the decrease and increase of the slip factor cause the corneringforce to increase and decrease, respectively.

If the braking force F_(x) for the front-left wheel FW_(L) is decreasedfrom the magnitude indicated by white arrow to the magnitude indicatedby black arrow, as shown in FIG. 46, therefore, the cornering forceF_(y) for the front-left wheel FW_(L) increases from the magnitudeindicated by white arrow to the magnitude indicated by black arrow. Ifthe braking force F_(x) for the rear-right wheel RW_(R) is increased asindicated by white and black arrows, on the other hand, the corneringforce F_(y) decreases from the magnitude indicated by white arrow to themagnitude indicated by black arrow. Thus, the smaller the braking forceF_(x) on the front-left wheel FW_(L), the more heavily the corneringforce F_(y) acts on the wheel. The greater the braking force F_(x) onthe rear-right wheel RW_(R), on the other hand, the smaller thecornering force F_(y) on the wheel is. As a result, the vehicle issubjected to the turning moment M(+) in the direction of its turn. InFIG. 46, hatched arrows indicate variations ±ΔF_(x) and ±ΔF_(y) of thebraking force F_(x) and the cornering force F_(y).

The inlet and outlet valves 12 and 13 for the front-left and rear-rightwheels FW_(L) and RW_(R), a diagonal pair of vehicle wheels, are openedand closed in accordance with the actual actuation mode M_(EXE) (i) andthe actual pulse width W_(EXE) (i) set on the basis of the required yawmoment γ_(d), so that the turning moment M(+) can be applied properly tothe vehicle. Thus, the tendency of the vehicle to understeer can beremoved, so that the vehicle can be prevented from drifting out.

The increase amount and decrease amount of the brake pressure for thefront-left wheel FW_(L) and the rear-right wheel RW_(R) are computed onthe basis of the same required yaw moment γ_(d), so that the absolutevalues of the increase amount and decrease amount are the same.Therefore, even if the brake pressures for the front-left and rear-rightwheels FW_(L) and RW_(R) are decreased and increased, respectively, thebraking force on the entire wheels does not change, and the brakingfeeling of the vehicle is not deteriorated.

Since the required yaw moment γ_(d) is computed in consideration of theoperating conditions and manipulations of the vehicle (see Steps S504and S505 in the computation routine of FIG. 11), fine yaw moment controlcan be effected according to the way the vehicle turns by increasing ordecreasing the wheel brakes on the diagonal pair of wheels in accordancewith the required yaw moment γ_(d).

The required yaw moment γ_(d) is computed on the basis of the yaw ratedeviation Δγ and the derivative Δγ_(s) of yaw rate deviation, so thatthe computed required yaw moment γ_(d) exactly indicates the turningbehavior of the vehicle at that time. If the braking force on a diagonalpair of vehicle wheels is increased or decreased according to therequired yaw moment γ_(d), therefore, unstable turning behavior of thevehicle is eliminated immediately, so that the vehicle can turn verystably.

In computing the required yaw moment γ_(d), open control according tothe lateral G_(Y) or the vehicle speed V and steering angle δ may beused, without using the aforesaid yaw rate feedback control.

Since the turn direction of the vehicle is determined based on theoutput of the yaw rate sensor 30, the turn direction of the vehicle canbe determined with a high accuracy, so the yaw moment control is carriedout accurately.

When the aforesaid yaw moment control is being executed and the vehicleis being braked, the actual actuation modes M_(EXE) (i) for the inletand outlet valves 12 and 13 for the front-right wheel FW_(R) andrear-left wheel RW_(L) are set to the noncontrol mode. Therefore, thecutoff valve 20, which is associated with the wheel brakes for thefront-right wheel FW_(R) and rear-left wheel RW_(L), is left open.Therefore, the wheel brakes for the front-right wheel FW_(R) andrear-left wheel RW_(L) can be subjected to the master cylinder pressure,so that the brake pressures of the front-right wheel FW_(R) andrear-left wheel RW_(L) are controlled by the operation of the brakepedal 3 performed by the driver. Consequently, the brake pressures ofthe front-right wheel FW_(R) and rear-left wheel RW_(L) are controlledby the driver's intention, and a fail-safe function for yaw momentcontrol is fully assured.

When the vehicle is not braked during the execution of yaw momentcontrol, the actual actuation modes M_(EXE) (i) for the inlet and outletvalves 12 and 13 for the front-right wheel FW_(R) and rear-left wheelRW_(L) are compulsorily changed into the hold mode, so that both theinlet and outlet valves 12 and 13 are closed (see Step S1008 in theactuation routine of FIG. 43).

Even if the pump 16 Is actuated by the motor 18 at this time, therefore,the discharge pressure of the pump 16 is not applied to the wheel brakesfor the front-right wheel FW_(R) and rear-left wheel RW_(L) via theinlet valve 12, so that the brake pressures of the front-right wheelFW_(R) and rear-left wheel RW_(L) are not increased undesirably.

Since the brake of the front-left wheel FW_(L) is not raised when thevehicle is not braked, it is substantially impossible, in this case, toreduce the brake pressure of the front-left wheel FW_(L), so that theturning moment M(+) to be given to the vehicle runs short. When thevehicle is not braked, however, as mentioned before, the required yawmoment γ_(d) is increased in computing it, so that the brake pressure ofthe rear-right wheel RW_(R) is increased more greatly than when thevehicle Is braked. Therefore, as the slip factor of the rear-right wheelRW_(R) increases, the cornering force F_(y) of the rear-right wheelRW_(R) decreases further. As a result, the cornering force of thefront-left wheel FW_(L) acts relatively strongly, so that the turningmoment M(+) as equal as when the vehicle Is braked is given to thevehicle.

When the driver depresses the brake pedal 3 at a speed higher than apredetermined pedal stroke speed (50 mm/s) during the execution of yawmoment control, the augmented depression flag F_(PP) of the brake pedal3 is set at 1 as explained regarding the setting routine of FIG. 6. Inthis case, in the forced-modification section 111 (see FIG. 23), theswitch 116 (see FIG. 35) is shifted from the shown position, so that theactuation modes M_(y) (i) of all wheels are compulsorily changed intothe noncontrol mode.

Therefore, the request flags F_(MON) and F_(COV) (i) are both reset to 0(see FIG. 36), and the actuation flags F_(VD1) (F_(V1)) and F_(MTR)(F_(M)) are also reset to 0 (see FIGS. 37 and 38). Thereupon, the cutoffvalve 19 is opened, and on the other hand, the actuation of the motor 18is stopped. The inlet valve 12 for each wheel is opened, and the outletvalve 13 therefor is closed. In this case, in the actuation routine ofFIG. 43, Step S1003 on the intensify-pressure mode is executed, so thatthe wheel brake for each wheel can be supplied with the master cylinderpressure. Therefore, the brake pressure in accordance with thedepression of the brake pedal 3 by the driver is raised in the wheelbrake for each wheel, by which the braking force on the vehicle can beassured fully.

Clockwise turn of oversteer-prone vehicle:

If the results of determination in Steps S602 and S604 in the selectionroutine of FIG. 26 are No and Yes, respectively, the vehicle has amarked tendency to oversteer. In this situation, unlike the aforesaidcase of the tendency to understeer, the intensify-pressure andreduce-pressure modes are established as the control mode M(1) and M(4)for the front-left wheel FW_(L) and the rear-right wheel RW_(R),respectively (see Table 1 and Step S605).

When the vehicle is braked, the braking force F_(x) and cornering forceF_(y) for the front-left wheel FW_(L) increases and decreases,respectively, while the forces F_(x) and F_(y) for the rear-right wheelRW_(R) decreases and increases, respectively, as shown in FIG. 47. Inthis case, therefore, the restoration moment M(-) is given to thevehicle. The restoration moment M(-) serves to remove the tendency ofthe vehicle to oversteer, thereby reliably preventing spinning of thevehicle attributable to a tack-in.

In the situation in which the clockwise turn of the vehicle isoversteer-prone, when the vehicle is not braked, or when the augmenteddepression flag F_(PP) is set at 1, the operation similar to that in thecase of understeer is achieved.

When clockwise turn of vehicle is not understeer and not oversteer:

In the selection routine of FIG. 26, when the results of determinationin Steps S602 and S604 are both No, and when the turning tendency of thevehicle is neither understeer nor oversteer, the pressure-hold mode isestablished as the control modes M(1) and M(4) for the front-left wheelFW_(L) and the rear-right wheel RW_(R), respectively (see Table 1 andStep S606).

In this case, the inlet and outlet valves 12 and 13 for the front-leftwheel FW_(L) and the rear-right wheel RW_(R) are both closed. Therefore,the brake pressures of the front-left wheel FW_(L) and the rear-rightwheel RW_(R) are kept. Neither the turning moment M(+) nor therestoration moment M(-) is given to the vehicle.

Counterclockwise turn of vehicle:

When the aforementioned turn flag F_(d) and on-off flag F_(ymc) are setat 1, the yaw moment control is executed for a counterclockwise turn ofthe vehicle. In this case as well, the turning moment M(+) is given tothe vehicle in the case where the vehicle has a marked tendency toundersteer, as in the aforementioned case of the clockwise turn. If thevehicle has a marked tendency to oversteer, on the other hand, the brakepressures of the front-right and rear-left wheels FW_(R) and the wheelRW_(L) are controlled in order to give the restoration moment M(-).Thus, even when the vehicle is turned counterclockwise, the same effectfor the case of the clockwise turn can be obtained (see Table 1 andSteps S607 to S611 in FIG. 26 and actuation routine of FIG. 43).

Countersteer of vehicle:

In a countersteer state in which the advancing direction of vehicle(solid line arrow mark: yawing direction) differs from the advancingdirection intended by the driver (broken line arrow mark: operatingdirection of steering wheel), as shown in FIG. 48, when the vehicle isnot braked, that is, in the case where the driver himself also requiresthe restoration moment of the vehicle, the values of turn directionflags F_(dy) and F_(s) do not agree with each other in the turndetermination routine of FIG. 8. In this case, a countersteer flagF_(cs) showing the countersteer state is set at 1 (Step S314).

In such a situation, even if the turn direction of the vehicle isdetermined based on the output of the yaw rate sensor 30, it isconcluded that the turn direction of the vehicle is counterclockwise,and the control execution flag F_(cos) is set at 1 (see Table 1 andselection routine of FIG. 26). In this case, the braking force on thefront-right wheel FW_(R), which is the outside wheel viewed in thevehicle turn direction, is increased. Therefore, the restoration momentM(-) is given to the vehicle, so that the vehicle can be turned stably.Since the vehicle is not braked, the reduction in pressure for therear-left wheel RW_(L) is not executed.

However, when the vehicle turns while being braked, and further thevehicle is in the critical braking condition such that the brakepressure using ABS is applied, since the slip factor of the front-rightwheel FW_(R) is already high, even if the brake pressure of thefront-right wheel FW_(R) is increased, that is, even if the slip factorof the front-right wheel FW_(R) is increased, the cornering force of thefront-right wheel FW_(R) is further decreased (see FIG. 45). As aresult, an effective restoration moment cannot be given to the vehicle.

When the front wheel is in the critical braking region, therefore, theresult of determination in Step S309 is Yes as shown in the turndetermination routine of FIG. 8, and the turn flag F_(d) is set based onthe steering wheel angle θ (Step S311). In this case, it is concludedthat even if the advancing direction (broken line arrow mark) of thevehicle is left, the turn direction is right (solid line arrow mark) asshown in FIG. 49.

When the turn direction of vehicle is determined in this manner, thepositive and negative of the yaw rate deviation Δγ are reversed asexplained in the description of the computation of the required yawmoment γ_(d), so that the control execution flag F_(cus), not theexecution control flag F_(cos), is set at 1. In this case, therefore, asseen from Table 1 and the selection routine of FIG. 26, the brakepressure of the front-left wheel FW_(L) is decreased, and the slipfactor thereof is decreased. Accordingly, as shown in FIG. 49, thecornering force F_(y) of the front-left wheel FW_(L) increases, so thatthe turning moment M(+) is given to the vehicle. This turning momentM(+) acts in the same direction as that of the restoration moment M(-)in FIG. 48, so that the vehicle is effectively subjected to therestoration moment as a consequence, by which the turn of vehicle can bestabilized.

According to Table 1 and the selection routine of FIG. 26, when thebrake pressure of the front-left wheel FW_(L) is decreased, the brakepressure of the rear-right wheel RW_(R) should be increased at the sametime. However, in the countersteer state, the increase in brake pressureat the rear-right wheel RW_(R) is inhibited. That is, when theaforementioned countersteer flag F_(cs) is set at 1, in the settingsection 94 in FIG. 29 (inhibitory section 90), the input conditions ofthe AND circuit 97 are met, so that the inhibiting flag F_(K1) (i)supplied from the AND circuit 97 to the switch 91 is set at 1, by whichthe switch 91 is shifted. In this case, therefore, the pulse widthW_(PLS) (4) of the rear-right wheel RW_(R) in the intensify-pressuremode is compulsorily changed to 0. Accordingly, even if the brakepressure control using ABS is carried out, the pulse width W_(PLS) (4)in the yaw moment control is outputted as a pulse width WW(4), and thebrake pressure of the rear-right wheel RW_(R) is not increased.

If the slip factor of the rear-right wheel RW_(R) is increased byincreasing the braking force thereof, the cornering force of therear-right wheel RW_(R) decreases. In this case, the increase in slipfactor at the rear-right wheel RW_(R) does not contribute at all to theaddition of the turning moment M(+), or exerts an adverse influence onit. In this case, however, since the increase in brake pressure at therear-right wheel RW_(R) is inhibited, the apparatus does not suffer theabove-mentioned disadvantage.

Excessive slip:

In the setting section 95 in FIG. 29 (inhibitory section 90), when thestate in which all inputs of the AND circuit are on is reached, that is,when the slip factor S_(L) (i) of a wheel in the intensify-pressure modebecomes higher than the allowable slip factor S_(LMAX) (i), theinhibiting flag F_(K2) (i) supplied from the AND circuit 98 to theswitch 92 is set at 1, so that the switch 92 is shifted. As a result,the pulse width W_(PLS) (i) is compulsorily changed to 0. Therefore,with the execution of the yaw moment control, the braking force on awheel in the intensify-pressure mode is increased. Consequently, if theslip factor exceeds the allowable value, the braking force on the wheelin not increased further. Thereupon, an excessive slip does not occur onthe wheel, and the brake pressure control using ABS is not carried outundesirably.

The allowable slip factor S_(LMAX) (i) is set based on the required yawmoment γ_(d) as shown in FIG. 32, so that in the state in which therequired yaw moment γ_(d) is large, and the vehicle strongly requiresthe yaw moment control, the inhibiting flag F_(K2) (i) is less prone tobe set at 1. Therefore, the increase in brake pressure is not inhibitedundesirably at the wheel in the intensify-pressure mode, so that the yawmoment control can be executed effectively.

On the other hand, as the yaw moment control is executed, the brakepressure of the wheel is continuously controlled in theintensify-pressure mode, so that the brake pressure control using ABS issometimes started on the wheel. In this case, the maximum value of theallowable slip factor S_(LMAX) (i) is set at a slip factor of vehicle atthe time when the brake pressure control using ABS is started, that is,the determination slip factor S_(LST) (i) (or 95% of S_(LST) (i)), andthe increase rate thereof is also set based on the new maximum value(see setting routine for inhibiting flag F_(K2) (i) in FIG. 31).Therefore, the vehicle locking tendency is eliminated by ABS, and evenif the vehicle control has been restored from ABS to yaw moment control,the intensify-pressure mode of vehicle is inhibited in the subsequentyaw moment control. Accordingly, the wheel does not reach the lockingtendency again, or the control is not changed frequently between brakepressure control using ABS and yaw moment control.

Cooperation with ABS:

It is assumed that when the ABS is operated and the brake pressure ofeach wheel is controlled based on the aforesaid actuation mode M_(ABS)(i) and pulse width W_(ABS) (i), the vehicle turns clockwise and theturn has an understeer tendency as shown in FIG. 50. In this case, inaddition to two wheels subjected to control in the yaw moment control,that is, the front-left wheel FW_(L) and rear-right wheel RW_(R), thefront-right wheel FW_(R) is also subjected to control, and thisfront-right wheel FW_(R) is controlled in the reduce-pressure mode.

When the brake pressure using ABS is carried out on the rear-right wheelRW_(R), the increase in braking force F_(x), at the rear-right wheelRW_(R), that is, the decrease in cornering force F_(y) cannot beexpected. However, if the cornering force F_(y) of the front-right wheelFW_(R) increases with the decrease in the braking force thereof, theturning moment M(+) can fully be given to the vehicle mainly based onthe difference in the cornering force F_(y) between the front and rearof the vehicle.

When the vehicle turns clockwise and the turning tendency is oversteeras shown in FIG. 51, the rear-left wheel RW_(L) is also controlled inaddition to the front-left wheel FW_(L) and the rear-right wheel RW_(R),which are subjected to control in yaw moment control, and the rear-leftwheel RW_(L) is controlled in the reduce-pressure mode. In this case,even if the decrease in the cornering force F_(y) at the front-leftwheel FW_(L) is not exhibited effectively by the brake pressure controlusing ABS, the restoration moment M(-) is fully given to the vehiclemainly based on the difference in cornering force F_(y) before and afterthe vehicle, like the aforementioned case.

Further, in the case where the rear-right and left wheels are set to bethe target wheels to be controlled in yaw moment control, when thevehicle turns clockwise and the turning tendency is understeer as shownin FIG. 52, the front-left wheel FW_(L) is also added as the targetwheel to be controlled, and the front-left wheel FW_(L) is controlled inthe reduce-pressure mode. Consequently, even if the increase in thebraking force on the rear-right wheel RW_(R) does not work due to thebrake pressure control using ABS, the cornering force F_(y) of thefront-left wheel FW_(L) is increased accordingly, so that the turningmoment M(+) can be given to the vehicle. When the vehicle turnsclockwise and the turning tendency is oversteer as shown in FIG. 53, therear-right wheel RW_(R) is added as the target wheel to be controlled,and the rear-right wheel RW_(R) is controlled in the reduce-pressuremode. In this case, even if the increase in braking pressure at thefront-left wheel FW_(L) cannot be achieved, the cornering force F_(y) ofthe rear-right wheel RW_(R) is increased accordingly, so that therestoration moment M(-) can be given to the vehicle.

What is claimed is:
 1. A turn control apparatus for a vehicle,comprising:determining means for determining a turning condition of thevehicle, said determining means including turn detecting means fordetecting a turn of the vehicle and outputting a turn signal indicativeof a turn direction of the vehicle and braking detecting means fordetecting the braking of the vehicle and outputting a braking signalindicative of a braking condition of the vehicle; selecting means forselecting the outside front and inside rear wheels viewed in the vehicleturn direction as two target wheels to be controlled based on said turnsignal from said turn detecting means; and first braking control meansfor increasing the braking force on one of said target wheels to becontrolled and decreasing the braking force on the other of said targetwheels to be controlled in accordance with said vehicle turningcondition determined by said determining means when the brakingdetecting means indicates the vehicle is being braked, wherein saidfirst braking control means sets the increase amount of braking force onsaid one of the target wheel to be controlled and the decrease amount ofbraking force on said other of the target wheel to be controlled at thesame absolute value.
 2. A turn control apparatus for a vehicle accordingto claim 1, wherein said braking detecting means includes augmenteddepression detecting means for detecting augmented depression of vehiclebrake pedal and outputting an augmented depression signal, andsaid firstbraking control means includes a hydraulic pressure control valves whichare changed over from the noncontrol position to control the brakepressure of the corresponding wheel brake by cooperating with the wheelbrake for each wheel, respectively, and returning means for returningall of said hydraulic pressure control valves to the noncontrol positionwhen said augmented depression signal is received.
 3. A turn controlapparatus for a vehicle according to claim 1, wherein said determiningmeans further includes vehicle condition detecting means for detecting avehicle operating condition and manipulation condition, andsaid firstbraking control means includes setting means for setting the increaseamount and decrease amount of braking force on said target wheels to becontrolled based on the detected vehicle operating condition andmanipulation condition.
 4. A turn control apparatus for a vehicleaccording to claim 3, wherein said vehicle condition detecting meansincludes means for setting a target yaw rate of the vehicle, andsaidsetting means of said first braking control means sets the increaseamount and decrease amount of braking force on said target wheels to becontrolled based on a yaw rate deviation between said target yaw rateand the actual yaw rate of the vehicle.
 5. A turn control apparatus fora vehicle according to claim 4, wherein said setting means of said firstbraking control means sets the increase amount and decrease amount ofbraking force on said target wheels to be controlled based on said yawrate deviation and the derivative of said yaw rate deviation.
 6. A turncontrol apparatus for a vehicle according to claim 1, wherein said turndetecting means includes a yaw rate sensor for detecting a vehicle yawrate and discriminating means for discriminating the vehicle turndirection based on the output of said yaw rate sensor.
 7. A turn controlapparatus for a vehicle according to claim 1, wherein said first brakingcontrol means includes a first brake line for supplying a brake pressureto wheel brakes for the front-left and rear-right wheels and a secondbrake line for supplying a brake pressure to wheel brakes for thefront-right and rear-left wheels.
 8. A turn control apparatus for avehicle according to claim 1, wherein said turn control turn controlapparatus further comprises second braking control means for increasingthe braking force on one of said target wheels to be controlled inaccordance with the vehicle turning condition determined by saiddetermining means when the vehicle turns while being not braked.
 9. Aturn control apparatus for a vehicle according to claim 8, wherein saiddetermining means further includes vehicle condition detecting means fordetecting a vehicle operating condition and manipulation condition,andsaid first and second braking control means include computing meansfor computing a controlled variable to be given to the vehicle, duringthe turn of vehicle, based on the vehicle operating condition andmanipulation condition detected by said vehicle condition detectingmeans, said computing means having a gain for computing said controlledvariable, which gain is set so as to be larger when the vehicle is notbraked than when the vehicle is braked, and setting means for settingthe increase amount and decrease amount of braking force on said targetwheels to be controlled based on the controlled variable from saidcomputing means.
 10. A turn control apparatus for a vehicle according toclaim 1, wherein said first braking control means includeshydraulicpressure control valves for controlling a brake pressure of wheel brakefor each wheel by cooperating with the corresponding wheel brake,respectively, a pump for supplying a pressurized fluid to the wheelbrake for each wheel, and holding means for changing over said hydraulicpressure control valve to hold the brake pressure of wheel brake for thewheel other than said target wheels to be controlled.
 11. A turn controlapparatus for a vehicle, comprising:determining means for determining aturning condition of the vehicle, said determining means including turndetecting means for detecting a turn of the vehicle and outputting aturn signal indicative of a turn direction of the vehicle and brakingdetecting means for detecting the braking of the vehicle and outputtinga braking signal indicative of a braking condition of the vehicle;selecting means for selecting the outside front and inside rear wheelsviewed in the vehicle turn direction as two target wheels to becontrolled based on said turn signal from said turn detecting means; andfirst braking control means for increasing the braking force on one ofsaid target wheels to be controlled and decreasing the braking force onthe other of said target wheels to be controlled in accordance with saidvehicle turning condition determined by said determining means when thebraking detecting means indicates the vehicle is being braked, whereinsaid braking detecting means includes augmented depression detectingmeans for detecting augmented depression of vehicle brake pedal andoutputting an augmented depression signal, and said first brakingcontrol means includes hydraulic pressure control valves which arechanged over from the noncontrol position to control the brake pressureof the corresponding wheel brake by cooperating with the wheel brake foreach wheel, respectively, and returning means for returning all of saidhydraulic pressure control valves to the noncontrol position when saidaugmented depression signal is received.